Sage Journals: Discover world-class research

Abstract

The effect of stearic acid-coated nano calcium carbonate (NCC) on transitions and chain mobility of poly(lactic acid) (PLA) was investigated. Dispersion state of NCC in polymeric matrix was explored using scanning electron microscopy and surface tension component measurements. Trends of PLA transitional phenomena were investigated using the results of dynamical mechanical analysis (DMA) and differential scanning calorimetry (DSC) in the nanocomposite systems based on PLA and NCC. In addition, two types of crystal structures and decreasing the glass transition temperature were distinguished using temperature-modulated DSC (TMDSC). Higher melting points of polymer crystals were found in TMDSC experiments due to low and dynamic heating rate compared to the conventional DSC. Dynamics of polymer chains, affected by NCC, were quantified using cooperativity length, ξ _α , and the number of relaxing structural units, N_α , in the glass transition region. NCC particles hindered the cooperative motion of polymer chains at glass transitions and crystallization in TMDSC measurements, whereas the DMA results indicated that NCC particles may act as lubricant and simplified chain mobility.

Keywords

Poly(lactic acid)nanocomposites glass transition molecular motion melting and crystallization

Introduction

Recently, there are many biodegradable plastics being used in the packaging applications but the objectives like their cost and performance factors are different from the conventional plastics used in packaging industries. In addition, the amount of usage will considerably decrease the cost; however, the challenges of performances are still comparable to the conventional plastics. Poly(lactic acid) (PLA) is made from 100% renewable resources and requires very less energy for production unlike the other biopolymers.^1

–4

Studies of polymeric nanocomposites based on PLA and hydroxyapatite, carbon nanotubes, fumed silica, and titanium dioxide have revealed that crystallinity, thermal stability, mechanical properties, gas barrier, degradation, and flame-retardant properties of PLA have been greatly improved.^5

–9 Recently, PLA/nano calcium carbonate (NCC)-based nanocomposites have received a lot of attention owing to its wide range of potential applications, improvement of mechanical properties, and production cost.^10

–13 Incorporation of NCC to PLA matrix increases tensile modulus and stiffness, and the resulting nanocomposite shows no brittle fracture behavior.^10,11 Calcium carbonate can be modified with different surfactants such as fatty acids, phosphonate, and titanate to improve the dispersion state in the polymer matrix.^14,15

Understanding of the thermodynamic properties of macromolecules, such as the biodegradable PLA, is in close association with molecular dynamics and can be quantitatively explored by thermal analysis.^16

–21 The occurrence of the glass transition in polymers is associated with cooperative motions of macromolecular chain segments. This structural relaxation is a kinetic phenomenon which arises from the nonequilibrium characteristic of the glassy state. The glass transition temperature (T _g), apparent heat capacity (C_p ), and enthalpy relaxation in the glass transition region depend on the rate of temperature change. The glass transitions involve the freezing of the cooperative, large-amplitude motion beyond the vibrational C_p which can be analyzed by modern calorimetry. Data from temperature-modulated differential scanning calorimetry (TMDSC) can be combined to obtain the apparent total, reversing, and non-reversing heat capacities.^22
–24

Adam and Gibbs²⁵ introduced cooperatively rearranging region (CRR) defined as a subsystem which can rearrange its configuration to another one independently of its environment.

Each CRR presents a subsystem having its own glass transition temperature and its own free volume, linked to its own relaxation time. According to Donth et al.,²⁶ the cooperativity length ξ _α at the glass transition corresponds to the size of a CRR. The CRR size is estimated using the temperature fluctuation of the amorphous medium. The cooperativity length ξ _α is associated with the volume of a CRR, $V α = ξ_{α}^{3}$ , and the number of relaxing structural units per CRR, N_α , also called the cooperativity degree, is estimated by using the following equations

V_{α} = ξ_{α}^{3} = \frac{Δ C_{p}^{- 1} k_{B} T_{α}^{2}}{ρ {(δ T)}^{2}}

N_{α} = \frac{N_{A} k_{B} T_{α}^{2} Δ C_{p}^{- 1}}{m_{0} {(δ T)}^{2}}

where $Δ C_{p}^{- 1}$ is the difference in the inverse of the isobaric heat capacity between the liquid and glass at T_α , T_α the dynamic glass transition temperature, ρ the density at T_α , k_B the Boltzmann constant, N_A Avogadro’s number, m ₀ the molar mass of the relaxing structural unit, and δT the temperature fluctuation in a CRR.

Another crucial transition of commercial PLA is cold crystallization phenomenon. Stiffness and modulus of PLA increase when the temperature is above about 90°C due to cold crystallization.²⁷ At the end of cold crystallization, melting process is started. Both the cold crystallization and melting of PLA depend on heating procedure with variables such as heating rate in DSC and TMDSC experiments and the measurement methods.²⁸

Recently, Bojda and Piorkowska²⁹ studied shear-induced nonisothermal crystallization of PLA. It was found that shearing intensified nonisothermal crystallization of PLA and all of the PLA transitions have been affected. The melting behavior of specimens reflected their thermomechanical history and the crystallinity changed during shearing, post-shearing, cooling, and subsequent heating, which led to single melting peaks with low or high temperature shoulders or to multiple melting peaks. Notta-Cuvier et al.³⁰ also investigated the effect of quasi-static and dynamic tensile tests on mechanical behaviors of PLA and PLA blends. It seems that different tension modes are important to characterize the properties of PLA.

The influence of crystallinity, nanofiller, and chain modification on the glass transition dynamics has been investigated in PLA,^14

–20 but to the best of our knowledge, the effects of rearranging time and applying tension during the experiment in solid state have been scarcely studied in the case of PLA-based systems especially in the presence of nanoparticles. In this study, chain rearranging of PLA during its transitions has been discussed and the effects of different methods were investigated in the solid state. DSC and TMDSC experiments were chosen as static tests (without tension) and dynamical mechanical analysis (DMA) was selected to be a dynamic test (with tension) for evaluation of PLA transitions in the presence of NCC as hindering, nucleating, and/or confining agent.

Experimental section

Materials

Amorphous PLA, grade 2002D, with Mn = 100 kg mol⁻¹ was kindly provided by NatureWorks Co. (USA). The acid-modified NCC, grade Hakuenka CCR-S, with rhombohedral shape, the average particle size of 80 nm and surface area of 17 m² g⁻¹, was obtained from Shiraishi-Omya (Austria) and used as received. The nano-sized precipitated calcium carbonate was coated with stearic acid in order to improve the dispersion in polymers.

Sample preparation

Melt compounding in the presence of nitrogen gas was employed to prepare the polymeric nanocomposites. The NCC was dried at 80°C for 18 h before melt mixing. PLA was melt compounded with NCC in a lab-scale internal mixer (Brabender®, Germany) at 180°C and rotor speed of 60 r min⁻¹ in order to prepare the nanocomposites with different NCC loading (0, 2, and 4 wt.% of NCC). The mixing time was 12 min (as an optimum value) to prepare samples with good dispersion state. Samples for DMA (with the size of 3 cm × 1 cm × 0.1 cm) and tensiometer (with the size of 2 cm × 2 cm × 0.1 cm) were prepared by hot-compression molding for 8 min at pressure and temperature of 10 MPa and 180°C, respectively.

Morphological properties

Scanning electron microscopy (SEM) of PLA/NCC nanocomposites was performed using a Seron Technologies Inc. SEM (Korea). All the samples were gold sputtered prior to examination.

Surface tension

Surface tensions (polar and dispersive parts) of PLA and its nanocomposites were measured using a tensiometer (kruss k-14, Germany). Surface tension parameters were determined by water and formamide as liquids in the plate method (according to Wilhelmy). Samples with sizes of (2 cm × 2 cm × 0.1 cm) were immersed vertically in the liquid and the surface tension components were obtained by measurement of the force acting on the plate.

Dynamical mechanical analysis

DMA characteristics of the obtained nanocomposite samples were evaluated using a DMA analyzer (Triton Co., TTDMA) under tension mode. The temperature sweep measurements were carried out in the range of 20°C to 100°C. The experiments were performed at a frequency of 1 Hz and at a heating rate of 2°C min⁻¹.

DSC and TMDSC experiments

Melting and crystallization characteristics of samples were examined using a METTLER/TOLEDO DSC and TMDSC systems. Samples with 11–13 mg weight were encapsulated in aluminum pans and treated at heating/cooling ramps (between 25°C and 200°C) at the rate of 10 and 2°C min⁻¹ in DSC experiments. The applied atmosphere was nitrogen with a flow rate of 80 mL min⁻¹. To evaluate the cold crystallization behavior of nanocomposites without the interference of thermal history, after the first run of heating, the cooling ramp (10°C min⁻¹) was performed to 25°C and subsequently, the second ramp of heating was employed. It should be noted that this type of PLA is not crystallized during the cooling from the melt.

The TMDSC experiments were performed on samples with the same thermal histories. Modulated temperature program was sinusoidal temperature profile with a heating rate of 2°C min⁻¹, the amplitude of 1°C, and modulation period of 1 min. Heat capacities of samples versus temperature were measured according to TMDSC experiments. All PLA relaxation would be identified more accurately using TMDSC and heat capacities compared to DSC thermogram. In the remaining parts, the abbreviated codes will be used for the obtained samples. The samples were coded as PLAxRyz. In this code, x is weight fraction of NCC, y is the heating rate of DSC or TMDSC experiment, and z indicate the DSC experiment (D) or TMDSC experiment (T).

Results and discussion

Dispersion state and surface tension of PLA and nanocomposites

Dispersion state of NCC performs an important role in transitions of PLA, for example, crystallization and melting phenomena. Besides, one of the key characteristics of PLA nanocomposites, especially in foam industries, that are noticeably affected by nanofiller dispersion, is the crystallization and nucleation behavior of the matrix.²¹

Morphology of PLA/NCC nanocomposites with different NCC contents is shown in Figure 1. As shown in this figure, nanocomposite containing 4 wt% of NCC (PLA4) has much more aggregates compared to nanocomposite containing 4 wt% of NCC (PLA2). In addition, the size of filler aggregation in PLA2 is smaller than that in PLA4.

Figure 1.

SEM micrographs of PLA2 (left images) and PLA4 (right images). The scale bars are 10 and 1 µm for top and bottom images, respectively. SEM: scanning electron microscopy; PLA: poly(lactic acid).

Because van der Waals interactions occur between all atoms and molecules, there is no substance with a surface energy/tension that solely consists of a polar part. On the other hand, there are substances that do not have any polar groups, such as alkanes that consist of hydrocarbon chains. Therefore, their surface energy/tension is purely dispersive.^31,32

By using the Wu method,³³ the solid surface free energy (Υ) and the corresponding polar (Υ _p ) and dispersive(Υ _d ) contributions were calculated. The solid surface energy of PLA without NCC (PLA0) is close to 41.6 mN m⁻¹, which can be separated into its two main component; the polar component (Υ _p ) with low value of about 10.8 mN m⁻¹ and the dispersive contribution (Υ _d ) with a relatively high value of 30.8 mN m⁻¹. Regarding the influence of the NCC content, a decreasing tendency is observed on surface free energy values as the NCC added to PLA as shown in Table 1. The surface energy of PLA4 is higher than that of PLA2 which can be described in terms of dispersion state of NCC in PLA2 sample. It seems that aggregation of NCC in PLA4 resulted in lower efficiency of NCC to decrease surface energy compared to PLA2 with a better dispersion state of nanofiller. The dispersive component of surface energy showed the same trend whereas the polar component value for PLA2 is more than that for PLA4. The polar contribution plays a key role in this overall increase in surface free energy as it changes from 10.8 mN m⁻¹ for PLA0 to 18.8 mN m⁻¹ for PLA2. Both NCC and PLA have polar groups in their structures besides the van der Waals interaction. Polar component of surface energy of NCC is much higher than PLA, so the addition of a small amount of NCC to PLA increased the Υ _p significantly.

Table 1.

Polar and dispersive components of surface energy for PLA and its nanocomposites.

Sample	Polar component of surface energy, Υ _p (mN m⁻¹)	Disperse component of surface energy, Υ _d (mN m⁻¹)	Surface energy of solid or Υ (mN m⁻¹)
PLA0	10.8	30.8	41.6
PLA2	18.8	14.8	33.6
PLA4	13.2	24.6	37.8

PLA: poly(lactic acid).

As mentioned earlier, before incorporation into plastics, calcium carbonate is often surface treated in order to facilitate dispersion in the polymer matrix and also to prevent loss of mechanical performance such as impact resistance. A common reagent is stearic acid. Stearic acid or its ammonium salt may be added directly during the precipitation step of calcium carbonate. Incorporation of stearic acid to calcium carbonate revealed a drastic decrease in surface energy, as a function of the degree of surface coverage by stearic acid. The totally covered calcium carbonate exhibits a surface energy close to that of pure stearic acid.³⁴ The NCC particles used in this work were modified with stearic acid and a layer of stearic acid was created around them. This is the reason for lower surface energy of nanocomposites compared to neat PLA. Besides the dispersion state and nucleation of NCC in polymeric matrix, stearic acid as a lubricant may decrease the polymer chain mobility and change the dependent properties.

Static thermal analysis

Crystallization and melting behavior of PLA and PLA/NCC nanocomposites in conventional DSC procedure with heating rates of 10 and 2°C min⁻¹ are shown in Figures 2 and 3, respectively. The heating rate of 2°C min⁻¹ was chosen to compare the results of conventional DSC and TMDSC. The characteristics of these curves including glass transition temperature (T _g), melting and cold crystallization temperatures (T _m and T _cc), the onset temperature of cold crystallization (T _ccs), and melting and cold crystallization enthalpies (ΔH _m and ΔH _cc) are summarized in Table 2.

Figure 2.

DSC heating curves of neat PLA and its nanocomposites filled with NCC after erasing thermal history; heating rate: 10°C min⁻¹. DSC: differential scanning calorimetry; PLA: poly(lactic acid); NCC: nano calcium carbonate.

Figure 3.

DSC heating curves of neat PLA and its nanocomposites filled with NCC after erasing thermal history; heating rate: 2°C min⁻¹. DSC: differential scanning calorimetry; PLA: poly(lactic acid); NCC: nano calcium carbonate.

Table 2.

Parameters obtained from conventional DSC curves of PLA and its nanocomposites.

Sample	T _g (°C)	T _ccs (°C)	T _cc (°C)	ΔH _cc (J g⁻¹)	T _mα′ (°C)	T _mα (°C)	ΔH _m (J g⁻¹)
PLA0R10D	58	92	116	20.18	-	146.9	20.45
PLA2R10D	58	96	122	13.13	-	148.3	13.49
PLA4R10D	58	94	121	14.63	-	148.3	14.91
PLA0R2D	55	82	96	29.91	143.5	150.9	27.17
PLA2R2D	55	87	97	29.68	143.7	150.9	27.80
PLA4R2D	55	85	96	28.21	143.5	150.8	25.65

DSC: differential scanning calorimetry; PLA: poly(lactic acid), T _mα′and T _mα are the melting temperatures of α and α′ crystals respectively.

Cold crystallization phenomenon can be observed for all samples due to rearrangement of polymer chains above glass transition. The enthalpy of cold crystallization was further found almost identical to the heat of fusion of the specimens in the experiments with heating rate of 10°C min⁻¹ (Table 2), suggesting that PLA did not crystallize at any specimens under this procedure. In other words, there is no crystallization during the cooling (cooling curves not presented). Therefore, the unique structural gradient formed in some slowly crystallizing polymers did not appear in PLA nanocomposites. The similar values of ΔH _cc and ΔH _m for each sample also indicated that the PLA was primarily amorphous. It seems that NCC nanoparticles would act as hindrance agents for crystallization so the cold crystallization phenomenon of PLA was influenced on contact with NCC. According to Figure 2 and Table 2, addition of NCC in PLA did not result in a noticeable change in the glass transition of PLA (about 58°C) due to low efficiency of NCC in confining polymer chains. It was found that the cold crystallization temperature of the neat PLA was clearly increased by the incorporation of NCC at 2 and 4 wt%, indicating hindered crystallization ability of PLA in the presence of NCC, which might behave as hindering agents in conventional DSC procedure. These trends were observed in Figure 3, where experiments were carried out with heating rate of 2°C min⁻¹ but the values of parameters were changed. While polymer chains have enough time to rearrange, the phenomena (glass transition or cold crystallization) are seen at lower temperatures. According to Tables 2, the T _g and T _cc values for samples exposed to heating rate of 2°C min⁻¹ are lower than heating rate of 10°C min⁻¹. In addition, unlike the heating rate of 10°C min⁻¹, samples are semicrystalline (due to the difference in melting and crystallization enthalpy) at the heating rate of 2°C min⁻¹ because more stable crystals during the heating were formed.

Results of TMDSC experiments were extracted and shown in Figures 4 to 6 and summarized in Table 3. Effect of NCC on PLA chain dynamics in glass transition region is demonstrated in these figures more accurately in the form of total heat flow and inphase and out-of-phase heat capacity of PLA and its nanocomposites. It seems that TMDSC with the special procedure is a useful tool to determine the chain dynamics of PLA. According to the out-of-phase C_p plots and Table 3, the dynamic glass transition (T _α) of PLA was increased significantly in the presence of NCC nanoparticles. Time dependencies of glass transition phenomena, cold crystallization phenomena, and melting phenomena of PLA have been reported in literature.^28,35,36 It is observed that these phenomena are more sensitive using the TMDSC experiment procedure compared to conventional DSC, especially with heating rate of 10°C min⁻¹. PLA chains are sensitive to rate and frequency of heating, so as in Figure 4, cold crystallization temperature for all samples decreased to the much lower temperature compared to conventional DSC. PLA polymer chains have more time to rearrange themselves in TMDSC procedures resulted in more uniform and stable crystals during the heating. Different crystal structures have been reported for PLA in which the crystal formation depends on the crystallization conditions. The most common α-form occurring in conventional melt and solution crystallization conditions was first reported by De Santis and Kovacs³⁷ and investigated further in a number of studies.^18,35,38,39 Based on wide angle X-ray diffraction (WAXD) and infrared (IR) data, Zhang et al. reported the slightly different α′-form for PLA crystallized below 120°C.⁴⁰ The chain conformation and crystal system of the α′-form is similar to α structure, but with a looser and less ordered chain packing. More recent studies suggest that only the α′ crystal is formed at crystallization temperatures below 100°C while crystallization between 100°C and 120°C gives rise to the coexistence of α′ and α crystal structures.⁴¹ As mentioned earlier, in TMDSC procedure and conventional DSC with heating rate of 2°C min⁻¹, PLA chains have enough time to rearrange and create α′ crystals besides α crystals. Melting phenomena of these two types of crystals are seen in Figure 4. Aforementioned α′ crystals showed lower melting point compared to α crystals as discussed earlier. Unlike to the conventional DSC with heating rate of 10°C min⁻¹, NCC particles did not affect cold crystallization temperature and melting phenomenon of the crystals in TMDSC experiment. It seems that in TMDSC, transitions of PLA chains can be evaluated using kinetic parts (e.g. non-reversing and out of phase heat capacity). In TMDSC and DSC with heating rate of 2°C min⁻¹ PLA chains have enough time to nucleate and grow the crystals so the effect of NCC is negligible in crystal structures. Another important factor which extracts from TMDSC experiment is heat capacity. As shown in Figures 5 and 6, the changes of heat capacities around glass transition and melting points were explored. As an interesting result, the T _α, which is the kinetic part of TMDSC experiments, was found to be higher than T _g (obtained by DSC) and affected by NCC loadings unlike to conventional DSC. Addition of NCC nanoparticles in PLA decreases the heat capacity differences around these two transitions (ΔC_p ). The changes of the out of phase heat capacity during the glass transition and melting for PLA4 sample is much lower compared to two PLA2 and PLA0. It may be the results of stearic acid coated on NCC which may create more interface area with PLA, especially during the melting.

Figure 4.

Total heat flow of neat PLA and its nanocomposites versus temperature in TMDSC heating profile. PLA: poly(lactic acid); TMDSC: temperature-modulated differential scanning calorimetry.

Figure 5.

Inphase heat capacity of neat PLA and its nanocomposites versus temperature in TMDSC heating profile. PLA: poly(lactic acid); TMDSC: temperature modulated differential scanning calorimetry.

Figure 6.

Out-of-phase heat capacity of neat PLA and its nanocomposites versus temperature in TMDSC heating profile. PLA: poly(lactic acid); TMDSC: temperature modulated differential scanning calorimetry.

Table 3.

Parameters obtained from TMDSC curves of PLA and its nanocomposites.

Sample	T _α (°C)	T _ccs (°C)	T _cc (°C)	ΔH _cc (J g⁻¹)	T _mα′ (°C)	T _mα (°C)	ΔH _m (J g⁻¹)
PLA0R2T	58.6	80	90.5	21.12	143.2	151.7	30.78
PLA2R2T	60.7	82	92.9	26.95	144.7	152.9	28.45
PLA4R2T	62.1	82	93.2	26.78	145.5	153.7	28.11

TMDSC: temperature-modulated differential scanning calorimetry; PLA: poly(lactic acid), T _mα′and T _mα are the melting temperatures of α and α′ crystals respectively.

Dynamics of polymer chains around the glass transition can be evaluated by measuring the cooperativity length (equation (1)) and the number of relaxing structural units per CRR (equation (2)). The average CRR sizes are calculated by taking ρ = 1.25 g cm⁻³, m₀ = 72 g mol⁻¹ for the PLA amorphous phase, and the temperature fluctuation δT is the half-width at half-height of the out of phase C_p spectra. As shown in Figure 7, the CRR size (ξ _α ) and degree of cooperativity (N_α ) decrease to a lower value in the presence of NCC. Results indicated that addition of NCC to PLA decreased chain flexibility leading to decrease in the size of cooperativity and the number of relaxing units.

Figure 7.

Variations in cooperativity length and cooperativity degree (number of relaxing units) versus NCC content. NCC: nano calcium carbonate.

Dynamic thermal analysis

Typical dynamic mechanical spectra of storage modulus, loss modulus, and loss factor (tan δ) for various PLA/NCC nanocomposites were recorded in the range of 20–130°C (Figures 8 to 10). As shown in Figure 8, below the glass transition temperature, storage modulus of PLA was decreased with addition of NCC. It seems that poor interphase properties of PLA and NCC particles and existence of defects in nanocomposite deteriorate the reinforcement effect of NCC. This phenomenon was reported before in the case of PLA/clay-based nanocomposites.^10,42 In addition, the presence of stearic acid around the nanoparticles during the modification process performs an important role to determine the mechanical properties of nanocomposites. Stearic acid may act as a lubricant and affect the mobility of the polymer chains. The results of dynamic measurement are discrepant with conventional DSC and TMDSC as a static measurement especially in the region of glass transition phenomenon. Results of conventional DSC showed no variation in the T _g of PLA and its nanocomposites whereas increase in the T _g values were observed in TMDSC experiments. It seems that the constraining effect of NCC was observed just in this experiment. According to DMA results, near the glass transition temperatures, opposing effect of static and dynamic measurements can be seen clearly. The DMA results demonstrated that addition of NCC to PLA decreased the T _g values, which may be the role of stearic acid coated on NCC. In other words, the plasticizing effect in the interfacial region offset the reinforcing effect of NCC, which attributes to the decrease in the T _g for the PLA4 sample. In addition, the tan δ peak height or area, an indication of the sample toughness, increases slightly when 4 wt.% of NCC is added into the PLA matrix, yet, in contrast, decreases as the 2 wt.% of NCC is added. It seems that in tension mode, dynamic tension increases the interface of PLA and stearic acid which may result in facilitating the chain mobility of PLA. Cold crystallizations of PLA and PLA/NCC nanocomposites were also seen in tan δ spectra (Figure 10). Similar to the effect of NCC on glass transition temperature, cold crystallization of PLA was accelerated by addition of NCC during the DMA experiments. It seems that the trends of both glass transition and cold crystallization of PLA in the presence of NCC are contrary to static tests (DSC and TMDSC). The reason was not understood completely but it may be the interface variation of PLA/stearic acid or rolling effect of NCC particles in tension mode of this dynamic test.

Figure 8.

Storage modulus versus temperature for PLA and its nanocomposites. PLA: poly(lactic acid).

Figure 9.

Loss modulus versus temperature for PLA and its nanocomposites. PLA: poly(lactic acid).

Figure 10.

Loss factor (tan δ) versus temperature for PLA and its nanocomposites. PLA: poly(lactic acid).

Conclusion

In this study, modified NCC was used to prepare nanocomposites based on PLA. This type of nanoparticle had different effect on chain mobility of PLA during the static (DSC and TMDSC) and dynamic (DMA) measurements. Glass transition, cold crystallization, and melting phenomena of PLA were not changed considerably using the conventional DSC procedures while in TMDSC procedures all of these phenomena were affected noteworthy by addition of NCC. In addition, in conventional DSC procedure, amorphous samples were obtained while in TMDSC experiments the final samples were semicrystalline. Opposing effect of dynamic measurements compared to static measurements was seen in the glass transition and cold crystallization regions for all neat polymer and nanocomposites which may attribute to lubricant effect of stearic acid coated on NCC.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Omid Yousefzade

References

Garlotta

. A literature review of poly(lactic acid). J Polym Env 2001; 9: 63–84. DOI: 10.1023/A:1020200822435.

Drumright

Gruber

Henton

. Polylactic acid technology. Adv Mater 2000; 12: 1841–1846. DOI: 10.1002/1521-4095(200012)12:23<1841:: AID-ADMA1841>3.0.CO;2-E.

Auras

Harte

Selke

. An overview of polylactides as packaging materials. Macromol Biosci 2004; 4: 835–864. DOI: 10.1002/mabi.200400043.

Armentano

Bitinis

Fortunati

. Multifunctional nanostructured PLA materials for packaging and tissue engineering. Prog Polym Sci 2013; 38: 1720–1747. DOI: http://dx.doi.org/10.1016/j.progpolymsci.2013.05.010.

Zheng

. Effect of surface modified hydroxyapatite on the tensile property improvement of HA/PLA composite. Appl Surf Sci 2008; 255: 494–497. DOI: http://dx.doi.org/10.1016/j.apsusc.2008.06.067.

Schvezov

Seligra

Lamanna

. International congress of science and technology of metallurgy and materials, SAM - CONAMET 2013PLA-fMWCNT bionanofilms with high modulus and great properties to apply in packaging and biomedicine. Procedia Mater Sci 2015; 8: 383–390. DOI: http://dx.doi.org/10.1016/j.mspro.2015.04.088.

Gong

Tian

Wen

. Synergistic effect of fumed silica with Ni2O3 on improving flame retardancy of poly(lactic acid). Polym Degrad Stab 2014; 104: 18–27. DOI: http://dx.doi.org/10.1016/j.polymdegradstab.2014.03.030.

Foruzanmehr

Vuillaume

Elkoun

. Physical and mechanical properties of PLA composites reinforced by TiO2 grafted flax fibers. Mater Des 2016; 106: 295–304. DOI: http://dx.doi.org/10.1016/j.matdes.2016.05.103.

Yuan

Qin

. Synthesis and characterization of Cerium-doped hydroxyapatite/polylactic acid composite coatings on metal substrates. Mater Chem Phys 2016; 182: 365–371. DOI: http://dx.doi.org/10.1016/j.matchemphys.2016.07.044.

10.

Jiang

Zhang

Wolcott

. Comparison of polylactide/nano-sized calcium carbonate and polylactide/montmorillonite composites: reinforcing effects and toughening mechanisms. Polymer 2007; 48: 7632–7644. DOI: http://dx.doi.org/10.1016/j.polymer.2007.11.001.

11.

de Oliveira

da Silva

ALN

Furtado de Sousa

. Composites based on green high-density polyethylene, polylactide and nanosized calcium carbonate: effect of the processing parameter and blend composition. Mater Chem Phys 2016; 181: 344–351. DOI: http://dx.doi.org/10.1016/j.matchemphys.2016.06.068.

12.

Jin

F-L

Pang

Q-Q

Zhang

T-Y

. Synergistic reinforcing of poly(lactic acid)-based systems by polybutylene succinate and nano-calcium carbonate. J Ind Eng Chem 2015; 32: 77–84. DOI: http://dx.doi.org/10.1016/j.jiec.2015.07.021.

13.

Kumar

Dev

Gupta

. Studies of poly(lactic acid) based calcium carbonate nanocomposites. Composites Part B: Eng 2014; 56: 184–188. DOI: http://dx.doi.org/10.1016/j.compositesb.2013.08.021.

14.

Badnore

Pandit

. Synthesis of nanosized calcium carbonate using reverse miniemulsion technique: comparison between sonochemical and conventional method. Chem Eng Process Processes Intensificat 2015; 98: 13–21. DOI: http://dx.doi.org/10.1016/j.cep.2015.10.003.

15.

Tran

. Facile surface modification of nanoprecipitated calcium carbonate by adsorption of sodium stearate in aqueous solution. Colloids Surf A Physicochemical Eng Aspects 2010; 366: 95–103. DOI: http://dx.doi.org/10.1016/j.colsurfa.2010.05.029.

16.

Salmerón Sánchez

Gómez Ribelles

Hernández Sánchez

. On the kinetics of melting and crystallization of poly(l-lactic acid) by TMDSC. Thermochimica Acta 2005; 430: 201–210. DOI: http://dx.doi.org/10.1016/j.tca.2005.01.066.

17.

Zuza

Ugartemendia

Lopez

. Glass transition behavior and dynamic fragility in polylactides containing mobile and rigid amorphous fractions. Polymer 2008; 49: 4427–4432. DOI: http://dx.doi.org/10.1016/j.polymer.2008.08.012.

18.

Delpouve

Saiter

Mano

. Cooperative rearranging region size in semi-crystalline poly(l-lactic acid). Polymer 2008; 49: 3130–3135. DOI: http://dx.doi.org/10.1016/j.polymer.2008.04.045.

19.

Yousefzade

Hemmati

Garmabi

. Thermal behavior and electrical conductivity of ethylene vinyl acetate copolymer/expanded graphite nanocomposites: effects of nanofiller size and loading. J Vinyl Addit Technol 2016; 22: 51–60. DOI: 10.1002/vnl.21428.

20.

Yousefzade

Garmabi

. Development of a simple model to characterize the complex constrained polymer chains in polymer nanocomposites at the interphase of amorphous/semicrystalline ethylene vinyl acetate and nanosheets. J Compos Mater 2016; 51(2): 179–186. DOI: 10.1177/0021998316643578.

21.

Yousefzade

Hemmati

Garmabi

. Assisted heterogeneous multinucleation and bubble growth in semicrystalline ethylene-vinyl acetate copolymer/expanded graphite nanocomposite foams: control of morphology and viscoelastic properties. eXPRESS Polym Lett 2015; 9: 932–944. DOI: 10.3144/expresspolymlett.2015.84.

22.

Ren

Urakawa

Adachi

. Dielectric and viscoelastic studies of segmental and normal mode relaxations in undiluted poly(d, l-lactic acid). Macromolecules 2003; 36: 210–219. DOI: 10.1021/ma0212341.

23.

Pyda

Wunderlich

. Reversing and nonreversing heat capacity of poly(lactic acid) in the glass transition region by TMDSC. Macromolecules 2005; 38: 10472–10479. DOI: 10.1021/ma051611k.

24.

Wunderlich

. Reversible crystallization and the rigid–amorphous phase in semicrystalline macromolecules. Progress in Polymer Science 2003; 28: 383–450. DOI: http://dx.doi.org/10.1016/S0079-6700(02)00085-0.

25.

Adam

Gibbs

. On the temperature dependence of cooperative relaxation properties in glass-forming liquids. J Chem Phys 1965; 43: 139–146. DOI: http://dx.doi.org/10.1063/1.1696442.

26.

Donth

. The size of cooperatively rearranging regions at the glass transition. J Non-Cryst Solids 1982; 53: 325–330. DOI: http://dx.doi.org/10.1016/0022-3093(82)90089-8.

27.

Jiang

Wolcott

Zhang

. Study of Biodegradable Polylactide/Poly(butylene adipate-co-terephthalate) Blends. Biomacromolecules 2006; 7: 199–207. DOI: 10.1021/bm050581q.

28.

Bouapao

Tsuji

Tashiro

. Crystallization, spherulite growth, and structure of blends of crystalline and amorphous poly(lactide)s. Polymer 2009; 50: 4007–4017. DOI: http://dx.doi.org/10.1016/j.polymer.2009.06.040.

29.

Bojda

Piorkowska

. Shear-induced nonisothermal crystallization of two grades of PLA. Polym Test 2016; 50: 172–181. DOI: http://dx.doi.org/10.1016/j.polymertesting.2016.01.006.

30.

Notta-Cuvier

Bouzouita

Delille

. Design of toughened PLA based material for application in structures subjected to severe loading conditions. Part 1. Quasi-static and dynamic tensile tests at ambient temperature. Polym Test 2016; 54: 233–243. DOI: http://dx.doi.org/10.1016/j.polymertesting.2016.07.020.

31.

Slade

Hild

. Surface energies of nonionic surfactants adsorbed onto nylon fiber and a correlation with their solubility parameters. Textile Res J 1992; 62: 535–546. DOI: 10.1177/004051759206200907.

32.

Saito

Yabe

. Dispersion and polar force components of surface tension of some polymer films. Textile Res J 1983; 53: 54–59. DOI: 10.1177/004051758305300110.

33.

. Calculation of interfacial tension in polymer systems. J Polym Sci Part C Polym Symposia 1971; 34: 19–30. DOI: 10.1002/polc.5070340105.

34.

Papirer

Schultz

Turchi

. Surface properties of a calcium carbonate filler treated with stearic acid. Eur Polym J 1984; 20: 1155–1158. DOI: http://dx.doi.org/10.1016/0014-3057(84)90181-2.

35.

Guo

Liu

. Non-isothermal crystallization kinetics of poly (lactic acid)/modified carbon black composite. Polym Bull 2009; 62: 629–642. DOI: 10.1007/s00289-009-0047-x.

36.

Monnier

Delpouve

Basson

. Molecular dynamics in electrospun amorphous plasticized polylactide fibers. Polymer 2015; 73: 68–78. DOI: http://dx.doi.org/10.1016/j.polymer.2015.07.047.

37.

De Santis

Kovacs

. Molecular conformation of poly(S-lactic acid). Biopolymers 1968; 6: 299–306. DOI: 10.1002/bip.1968.360060305.

38.

Michalski

Makowski

Biedroń

. Controlling polylactide stereocomplex (sc-PLA) self-assembly: from microspheres to nanoparticles. Polymer 2016; 90: 242–248. DOI: http://dx.doi.org/10.1016/j.polymer.2016.03.049.

39.

Nofar

Zhu

Park

. Crystallization kinetics of linear and long-chain-branched polylactide. Ind Eng Chem Res 2011; 50: 13789–13798. DOI: 10.1021/ie2011966.

40.

Zhang

Duan

Sato

. Crystal modifications and thermal behavior of poly(l-lactic acid) revealed by infrared spectroscopy. Macromolecules 2005; 38: 8012–8021. DOI: 10.1021/ma051232r.

41.

Kawai

Rahman

Matsuba

. Crystallization and melting behavior of poly (l-lactic acid). Macromolecules 2007; 40: 9463–9469. DOI: 10.1021/ma070082c.

42.

Lai

S-M

S-H

Lin

G-G

. Unusual mechanical properties of melt-blended poly(lactic acid) (PLA)/clay nanocomposites. Eur Polym J 2014; 52: 193–206. DOI: http://dx.doi.org/10.1016/j.eurpolymj.2013.12.012.

Poly(lactic acid) phase transitions in the presence of nano calcium carbonate: Opposing effect of nanofiller on static and dynamic measurements

Abstract

Keywords

Introduction

Experimental section

Materials

Sample preparation

Morphological properties

Surface tension

Dynamical mechanical analysis

DSC and TMDSC experiments

Results and discussion

Dispersion state and surface tension of PLA and nanocomposites

Static thermal analysis

Dynamic thermal analysis

Conclusion

Footnotes

Declaration of Conflicting Interests

Funding

ORCID iD

References