Poly(l-lactic acid) Crystallization in a Confined Space Containing

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Poly (-lactic acid) Crystallization in a Confined Space Containing Graphene Oxide Nanosheets Hua-Dong Huang, Jia-Zhuang Xu, Ying Fan, Ling Xu, and Zhong-Ming Li J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp4055796 • Publication Date (Web): 20 Aug 2013 Downloaded from http://pubs.acs.org on August 22, 2013

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The Journal of Physical Chemistry

Poly (L-lactic acid) Crystallization in a Confined Space Containing Graphene Oxide Nanosheets

Hua-Dong Huang, Jia-Zhuang Xu, Ying Fan, Ling Xu, Zhong-Ming Li*

College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, P. R. China

*

Corresponding author. Tel.: +86-28-8540-6866; Fax: +86-28-8540-6866

E-mail address: [email protected] (Z.-M. Li)

1

ACS Paragon Plus Environment


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ABSTRACT: The semicrystalline polymer incorporated with nanofillers frequently exhibits complicated crystallization behavior, which is probably attributed to the nanofiller-constructed complex crystalline circumstance, especially a confined space. In the present work, in order to have a thorough understanding of biodegradable poly(L-lactic acid) (PLLA) crystallization behavior on the dependence of graphene oxide nanosheet (GONS) loadings, in particular the relatively high GONS loading, a set of GONS/PLLA nanocomposites with different GONS loadings ranging from 0 to 4.0 wt% were investigated in terms of isothermal crystallization behavior by differential scanning calorimetry and time-resolved fourier-transform infrared spectroscopy techniques. The results indicated that GONSs not only served as heterogeneous nucleating agents for PLLA crystallization, but also restricted the mobility and diffusion of PLLA chains. At low GONS concentrations of 0.25 wt% and 0.5 wt%, GONSs acted as a temple for PLLA chains to land on due to extremely high specific surface area, thus promoting the conformational ordering and reducing the nucleating barrier. Nucleation effect of GONSs was dominant to achieve accelerated overall crystallization kinetics. As the GONS concentration rose up to 1.0 wt%, GONS network was formed in PLLA matrix, which was verified by solid-like rheological behavior at low frequencies in rheological measurement. The nanofiller network significantly constrained the mobility and diffusion of PLLA chains and offset the nucleation effect of GONSs, giving rise to a turning point in crystallization rate from promotion to restriction. Furthermore, a severely confined space was constructed by the more crowded and denser GONS networks at a higher GONS 2


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concentration of 4.0 wt%, compelling PLLA lamellae to grow in a two-dimensional mode. The unusual crystallization behavior of PLLA from promotion to restriction was also understood by the four-region model, in which the semi-quantitatively description of crystalline circumstance was provided. These results pave an effective way to further reveal the crystallization behavior of polymer at a relatively high nanofiller loading.

KEY WORDS: crystallization kinetic, crystalline circumstance, four-region model, GONS network

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INTRODUCTION Polymer nanocomposites consisting of nano-sized fillers have been an area of active

research due to their potential to achieve high-performance and

multi-functional materials.1,2 A number of studies have been systematically performed focusing on the relationship between the microstructure and the macroscopic properties of polymer nanocomposites, especially for semi-crystalline polymer based ones, where their crystallinity and crystalline morphology play a critical role in determining the ultimate properties of these nanocomposites.3,4 For instance, the nanofiller-induced interfacial crystalline layer bridging the nanofiller and polymer matrix can effectively enhance the interfacial stress transfer, giving rise to a remarkable improvement on mechanical properties of polymer nanocomposites.5-7 Exploring the role of nanofiller in the polymer crystallization has thus become a crucial subject in the field of nanocomposites.8,9 Up to now, it is well established that nanofillers could be recognized as effective heterogeneous nucleating agents for various semi-crystalline polymers, thus affecting the

crystallization

kinetics

and

resultant

crystalline

morphology

of

nanocomposites.10,11 The excellent nucleating ability of nanofillers is generally attributed to their extremely high specific surface areas and large aspect ratios,12,13 which lowers the surface free energy barrier towards nucleation and initiate crystallization at high temperature. Hence, only a small amount of nanofillers could effectively accelerate the nucleation and overall crystallization rate of a polymer matrix, which is mainly manifested by the reduction of crystallization induction 4


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period, half-crystallization time and spherulite size.12-21 Meanwhile, nanofillers have a high geometrical effect for the epitaxial growth of polymer lamellar crystals, resulting in unique crystalline morphologies.20,22-24 e.g., a “nanohybrid shish kebab” (NHSK) was observed in carbon nanotubes (CNTs)/nylon 66 nanocomposites wherein the CNTs served as the shish while nylon 66 lamellar crystals formed the kebabs.24 Recently, a nanohybrid structure with polyethylene epitaxial crystals on both surfaces of reduced graphene

nanosheets was obtained via a controlled solution

crystallization procedure, in which polyethylene edge-on crystals formed from the randomly distributed rodlike nuclei on the basal plane of reduced graphene nanosheets and the c-axis of polyethylene chain was parallel to basal plane of reduced graphene nanosheets.20 For the purpose of highlighting the prominent nucleating effect of nanofillers, the nanofiller content was usually chosen to be as low as possible in the most previous research. However, to achieve high-performance polymer nanocomposites for practical applications, a relatively high nanofiller loading is required.25,26 In this scenario, a critical concentration exists at which the distance between nanofillers is so small that they could join together side by side, consequently maximizing the contribution to mechanical properties with greatest efficiency. Different from low nanofiller loading systems, the presence of high loading of nanofillers not only provides a large number of heterogeneous nucleating sites to accelerate crystallization rate, but also restricts the mobility and diffusion of polymer chains in front of crystal growth to bring about a higher nuclei barrier and activation of polymer chain 5



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diffusion, in particular when the nanofiller concentration is high enough to set up network structure.27,28. Thus, it is obvious that the influence of nanofillers with a relatively high loading on polymer crystallization is two-fold, which makes nanofiller-induced polymer crystallization behavior much more complicated than that at a low nanofiller loading.24 Meanwhile, such complicated polymer crystallization behavior is probably attributed to the nanofiller-constructed complex crystalline circumstance, and the quantitative or semi-quantitative description of such circumstance has not been achieved yet. Recently, graphene oxide nanosheets (GONSs), layered materials derived from graphene nanosheets, have sparked great excitement in carbon-based nanocomposites as versatile materials due to their unique merits.29 Moreover, GONSs with particular two-dimensional planer structure and amazing specific surface area, are recognized as an efficient nucleating agent to promote polymer crystallization and provide a good platform for polymer epitaxial crystallization.17,19,21,30 In contrast to graphene nanosheets, GONSs are heavily oxygenated, bearing abundant oxygen-containing functional groups on their basal planes and edges, such as hydroxyl, epoxide, carbonyl and carboxyl.31 These chemical functional groups have been found to be feasible and effective for promoting their complete exfoliation and uniform dispersion in polymer matrix, as well as enhancing interfacial bonding between GONSs and polar polymer matrix. Such interactions not only facilitate polymer chain arrangement in lattice and the corresponding mechanism can be interpreted in terms of surface-induced conformational ordering,17,19,21 but also restrict the movement of 6


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polymer chains, and consequently result in a decrease in crystallization kinetics of polymer matrix. Therefore, GONSs are promising alternative candidates to systematically

investigate

the

mechanism

of

nanofiller-induced

polymer

crystallization at a relatively high nanofiller loading via building the complex crystalline circumstance. Nevertheless, a full and generalized understanding of the effect of relatively high GONS loading on the crystallization behavior of polymer matrix in the nanocomposites has not received any attention. In the current study, poly (L-lactic acid) (PLLA), an increasingly popular biodegradable polymer originating from renewable resource, has been adopted as a model polymer to probe the effect of the differently dimensional nanofillers on the crystallization behavior of polymer nanocomposites incorporated with a low content of GONSs and CNTs, attributing to its relatively long induction crystallization time and low crystallization rate. Herein, in order to have a thorough understanding of PLLA crystallization behavior on the dependence of GONS loadings, in particular the relatively high GONS loading, a set of GONS/PLLA nanocomposites with different GONS loadings ranging from 0 to 4.0 wt% were fabricated. The obtained nanocomposites exhibit unusual crystallization kinetics from promotion to restriction with the increase of GONS loadings and the corresponding mechanism was also discussed via four-region model. The results fill the gaps of polymer crystallization induced by the high concentration of GONSs, and have guide significance for achieving high-performance and multi-functional polymer nanocomposites.

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EXPERIMENTAL SECTION Materials. Commercially available PLLA was purchased from Nature Work (4032D) with around 2% D-LA. The weight-average molecular weight and number-average molecular weight were 2.23×105 g/mol and 1.06×105 g/mol, respectively. GONSs were synthesized from expanded graphite by the modified “Hummers” method and the details of the preparation process has been reported in our previous work.17 Anhydrous ethanol (C2H5OH) and dichloromethane (CH2Cl2) were provided from Chengdu Kelong Chemical Reagent Factory, Chengdu, China. The materials were directly used without further purification. Preparation of GONS/PLLA Nanocomposites. Solution coagulation was proposed to prepare a series of GONS/PLLA nanocomposites containing various GONS loadings of 0.25, 0.5, 1.0, 2.0, and 4.0 wt%. Taking the 0.25 wt% GONS/PLLA nanocomposite as an example, the detailed synthesis procedures were as follows: 75 mg GONSs and about 600 ml C2H5OH solution were placed in a flask equipped with mechanical stirrer and dispersed in a ultrasonic bath for 3 h at room temperature to obtain a uniform suspension. 30 g PLLA granules were completely dissolved into about 300 ml CH2Cl2 solution with the aid of mild stirring at room temperature. Upon completion, the coagulation of GONS/PLLA nanocomposites was accomplished by adding the well-dispersed GONS/C2H5OH suspension dropwise into the PLLA/CH2Cl2 hybrid. Finally, the coagulated nanocomposites were isolated via filtration; washed with C2H5OH solution; left in a drying oven at 60 oC to remove bulk solvents; and dried in a vacuum oven overnight at 60 oC to evaporate the any 8


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residual solvent. For comparison purposes, neat PLLA were prepared according to the same procedures. Dispersion morphology of GONSs in the PLLA matrix. The field emission scanning electron microscopy (SEM) was performed on an FEI Inspect-F (Finland) with an acceleration voltage of 5 kV to study the dispersion morphology of GONSs in the PLLA matrix. The nanocomposite film with GONS loading of 2.0 wt% prepared by compressing molding at 180 oC using the coagulated nanocomposite, was cryo-fractured in liquid nitrogen and then coated with a thin layer of gold prior to being observation. Rheological Measurement. To study the rheological properties of GONS/PLLA nanocomposites as a function of GONS loading, an AR2000EX stress-controlled rotational rheometer (TA Instruments, USA) was employed using 25 mm diameter parallel-plate geometry with 800 µm gap setting under nitrogen atmosphere. Before the rheological measurement, all the disk-shape samples with a thickness of 1 mm and diameter of 25 mm were prepared by compressing molding at 180 oC. And the obtained samples were further dried in vacuum oven at 60 oC for 6h. Oscillatory frequency sweeps from 628 to 0.0628 rad/s with a fixed strain 1.0 % were applied at 180 oC for all GONS/PLLA nanocomposite samples. Differential

Scanning

Calorimetry

(DSC)

Characterization.

Overall

isothermal crystallization kinetic of all GONS/PLLA nanocomposite samples was investigated by DSC using a TA-Q200 instrument (USA). The experiments were carried out in nitrogen atmosphere using about 5 mg sample sealed in aluminium plan. 9



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The detailed isothermal crystallization procedure was designed as follows: all the samples were heated from 40 to 180 oC at a heating rate of 20 oC/min, held at 180 oC for 5 min to eliminate thermal history, and then quenched to 100 oC at a cooling rate of 30 oC/min for isothermal crystallization until no changes in the heat flow were observed. Heat flows during isothermal crystallization of PLLA nanocomposites with different GONS loadings were recorded. The relative crystallinity at time t, X(t) , is defined by the following equation: X c (t) X(t) = = X c (t ∞ )

dH(t) dt ∆H t dt = t∞ dH(t) ∆H ∞ ∫0 dt dt

∫

t

0

(1)

where dH dt is the rate of heat evolution, ∆H t is the heat generated at time t, and ∆H ∞ is the total heat by the end of isothermal crystallization process.

Time-resolved

Fourier-Transform

Infrared

Spectroscopy

(FTIR)

Characterization. Time-resolved FITR characterization was performed on a Nicolet 6700 spectrometer (Thermal Scientific, USA) equipped with a heated transmission cell (HT-32). In situ spectra were collected over the wavenumber range 4000 ~ 600 cm-1 by averaging 16 scans at a 2 cm-1 resolution with 1 min intervals, which has already been subtracted from the background spectra. In order to ensure that the films examined were thin enough to be within the absorption range, the sample thickness was controlled at about 10 µm by compressing molding. But it was still difficult for 4.0 wt% GONS/PLLA nanocomposite to obtain high-quality spectra without undetectable absorption peaks due to its very low IR transmittance so that it was absence during the in situ FTIR analysis. In the process of isothermal crystallization, 10


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all the GONS/PLLA nanocomposite films were sandwiched between two ZnSe plates so as to adopt the transmission mode. Considering that it is impossible to exactly differentiate the changes in the various vibrations of PLLA chains isothermally crystallized at a low temperature due to the mismatching between fast crystallization rate and slow IR spectra acquisition speed, the isothermal crystallization temperature was chosen to be 135 oC. Each sample was heated from room temperature to 200 oC at a heating rate of 30 oC/min, held at 200 oC for 5 min to erase thermal history, and then cooled down to 135 oC within less than 4 min for isothermal crystallization. When the experimental temperature reached at 135 oC, the data collection started until the completion of crystallization. The baseline of all spectra was linearly corrected according to the same standard, and the intensities of the characteristic bands were used to depict the conformational changes and crystallization kinetic of all the PLLA nanocomposite samples as a function of GONS loading. The intensity of characteristic bands is normalized by the following expression:

Ir =

I t - I0 × 100% I ∞ - I0

(2)

where I r is the relatively intensity. It is the peak intensity at the crystallization time t. I0 and I ∞ are respectively the initial and final peak intensities during isothermal crystallization.

RESULTS Dispersion morphology of GONSs in the PLLA matrix. Dispersion morphology of nanofillers in polymers often plays an important role in fabricating 11



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high performance of polymer-based nanocomposites.32,33 Figure 1 shows the typical SEM images for the fractured surfaces of 2.0 wt% GONS/PLLA film. It can be seen that GONSs pointed by green arrows are fully exfoliated and clearly well dispersed in PLLA matrix. Furthermore, GONSs are tightly embedded in the nanocomposite film, suggesting the excellent compatibility between GONSs and PLLA matrix. The phenomenon is possibly attributed to the hydrogen bonding between the oxygen-containing functional groups on GONS layers and ester groups on PLLA molecular chains, as well as the mechanical interlocking resulted from the special wrinkled structure of GONSs.34

Figure 1. Typical SEM images with two different resolution magnitudes for the fractured surfaces of 2.0 wt% GONS/PLLA film.

Isothermal Crystallization Kinetics of GONS/PLLA Nanocomposites. The isothermal crystallization kinetic analysis of GONS/PLLA nanocomposites was employed to reveal the effect of GONS on the crystallization behavior of PLLA. Figure 2 shows heat flow curves, relative crystallinity and Avrami plots of neat PLLA and its nanocomposites with varying GONS loadings during isothermal crystallization 12


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at 100 oC. It can be seen from Figure 2a that after a very short crystallization induction period, the exotherms drift away from the baseline, gradually reach the maximum peak along the time axis and then go back to the platform, which represents the heat response throughout PLLA crystallization process. With increasing GONS concentrations the peak time of exothermic heat flow curve initially shifts to lower values, but unexpectedly, when beyond 1.0 wt%, it shifts back to higher values compared with neat PLLA, exhibiting a very interesting crystallization behavior. This observation can be affirmed more clearly by the changes of half crystallization time (t0.5). According to Equation (1), relative crystallinity of neat PLLA and its GONS nanocomposites as a function of crystallization time is displayed in Figure 2b. t0.5, defined as the time taken from the onset of the crystallization until 50% completion of the crystallization process, is shown in Table 1. When only adding 0.25 wt% GONSs, t0.5 of the nanocomposite declined from 3.89 min to 3.17 min as compared with neat

PLLA, indicating the heterogeneous nucleating ability of GONSs. This acceleration effect of the nanofiller on crystallization kinetics of polymers is prevalent in most of nanocomposite systems, such as clay, CNTs, zinc oxide, etc.

14,16,18

As the GONS

loading rises to 0.5 wt%, there is only 4% reduction in t0.5 from 3.89 to 3.73 min and its enhancement on crystallization kinetics is inferior to that of 0.25 wt% GONS/PLLA nanocomposite, which infers that the nucleating effect of GONSs may saturate at a GONS loading of 0.25 wt%.27,28 Interestingly, with further increasing GONS loading to 1.0 wt%, the typical sigmoid curve shifts to a longer time region compared with neat PLLA, which implies that such a GONS loading inhibits, rather 13



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than promotes the overall crystallization kinetics of PLLA. There is therefore a GONS loading-induced transition of PLLA crystallization kinetics from promotion to restriction where t0.5 increases from 3.89 min to 4.07 min. The crystallization rate is more significantly depressed with further increasing the GONS content. About 74 % increase in t0.5 from 3.89 to 6.75 min appears upon addition of 4.0 wt% GONSs. The reasons for such an interesting crystallization behavior induced by different GONS loadings will be detailedly discussed in the Discussion section. The isothermal crystallization kinetic is often analyzed by the well-known Avrami equation where relative crystallinity develops with crystallization time as

lg[− ln(1 − X (t ))] = lgK + nlgt

(3)

where n is the Avrami exponent and K is the crystallization rate parameter, which is dependent on the nucleation and growth mechanism of crystallization.35,36 Figure 2c shows the Avrami plots of neat PLLA and its GONS nanocomposites isothermally crystallized at 100 oC, wherein a good linearity between lg[− ln(1 − X (t ))] and lgt is presented and the n and K can be obtained respectively from the slopes and the intercepts. The obtained Avrami parameters of neat PLLA and its nanocomposites are summarized in Table 1. As listed in Table 1, it can be seen that the n of all samples except 4.0 wt% GONS/PLLA nanocomposite is around 3.0, which implies that PLLA crystallizes mostly followed by three-dimensional spherulite growth in a heterogeneous nucleating mode. However, as the GONS loading rises to 4.0 wt%, the Avrami exponent is declined to only 2.2, suggesting a quasi-two-dimensional crystal growth 14


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mode. In the previous work, some reports on crystallization kinetics of polymer nanocomposites showed that the incorporation of nanofiller did not affect the n significantly.15,27 And some other reports showed that n decreased slightly with adding nanofiller into polymer matrix.24,37,38 The decreased n can be principally explained by the following reasons: (1) nanofillers, such as CNTs and graphene nanosheets, serve as templates for polymer crystal growth; (2) high nucleation density causes confined growth between the adjacent polymer crystals; (3) different experimental conditions may affect the dimension of polymer crystal growth. In the present case, only the sample with the highest content of GONSs exhibits obvious decline in n and the reason for this will be deeply elaborated in the Discussion section. In addition, note that the K of 4.0 wt% GONS/PLLA nanocomposite is the largest in all the samples, 1.08 × 10-2, revealing the fastest crystallization rate. Such result is not in line with its lowest crystallization rate characterized by DSC. At this point, it should be noted that the dimension of K is min-n, manifesting a close relationship between K and n. Thus, K of different samples cannot be compared with each other, attributing to their

different Avrami exponents.39-41 Herein, another representation of K is introduced as

K = k n , where k is also taken as crystallization rate parameter, but its dimension becomes min-1, which is independent on the Avrami exponent. Hence, k is reasonable to evaluate the crystallization rate of different samples in the study. As seen in Table 1, with the increase of GONS content, the value of k first increases and then decreases, displaying the same trend of t0.5 as described above.

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Figure 2. Heat flow curves (a), relative crystallinity (b) and Avrami plots (c) of neat PLLA and GONS/PLLA nanocomposites with different GONS loadings.

Table 1. Crystallization kinetic parameters for neat PLLA and GONS/PLLA nanocomposites with different GONS loadings GONS loadings (wt%)

K (10-2 × min-n)

k (min-1)

16


n

t0.5 (min)

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0

1.19

0.23

3.0

3.89

0.25

2.48

0.28

2.9

3.17

0.5

1.32

0.24

3.0

3.73

1.0

1.07

0.22

3.0

4.07

2.0

0.64

0.19

3.1

4.61

4.0

1.08

0.13

2.2

6.75

Conformational

Changes

of

GONS/PLLA

Nanocomposites

during

Isothermal Crystallization. FTIR is a powerful tool to reveal the structural and conformational changes of polymers during melt-crystallization or cold-crystallization, owing to its high sensitivity to the conformation and packing density of molecular chains.16,21,42 Herein, in situ FTIR measurement was carried out to further dissect the possible crystallization mechanism of PLLA nanocomposites. Figure 3a-e displays the time-resolved spectra during isothermal crystallization for neat PLLA and its GONS nanocomposites in the range of 1500 ~ 800 cm-1, in which the FTIR spectra are highly sensitive to the structural change during isothermal crystallization of PLLA.14,17,43 Both band shift and intensity changes of some characteristic peaks apparently occur with increasing crystallization time. In addition, the difference spectra obtained by the subtraction of initial spectrum from the consecutive spectra is another effective path to characterize the band shift and intensity changes, which can also recognize the crystalline-dependent bands. As shown in Figure 3a′-e′, it is noted in the difference spectra that the bands in the positive regions are crystalline-dependent while the 17



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amorphous-dependent bands are located in the negative regions. On the basis of previous research,16,17,43,44 a set of bands at 1458, 1210 and 921cm-1 are closely linked with the crystallization behavior of PLLA. The vibrational assignments of the three bands are summarized in Table 2. The peak at 1458 cm-1 is assigned to the CH3 asymmetric deformation mode (δas(CH3)), being representative of –CH3 inter-chain interaction. It has a slight intensity increase and peak shift during isothermal crystallization. The relatively large changes take place at the band of 1210 cm-1, which is identified as the C-O-C asymmetric vibrations (νas(C-O-C)) linked with asymmetric CH3 rocking vibrations (γas(CH3)), also being indicative of inter-chain conformational ordering. The two bands at 1458 and 1210 cm-1 are often used to probe the conformational changes in the backbone. The band at 921 cm-1 resulting from the couple of C-C backbone stretching and the CH3 rocking mode, is characteristic of PLLA α-crystals with the distorted 103 helix conformation. In the present case, because the 921 cm-1 is considered as the pure crystalline-specific band, its intensity changes are adopted to track the kinetics of 103 helix formation and to comparatively study the crystallization kinetics of neat PLLA and its GONS nanocomposites. At the initial stage of isothermal crystallization, no peak could be observed at 921cm-1, indicating complete elimination of thermal history (Figure 3a-e). In the course of crystallization, the intensity of 921 cm-1 gradually increases until the crystallization is completed.

18


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Figure 3. Time-resolved spectra and the corresponding difference spectra in the range 1500 ~ 800 cm-1 isothermally crystallizing at 135 oC for neat PLLA (a, a′) and its GONS nanocomposites as a function of GONS loadings, 0.25 wt% (b, b′), 0.5 wt% (c, c′), 1.0 wt% (d, d′), and 2.0 wt% (e, e′).

Table 2. Band assignments of semi-crystallization of PLLA14,16,43 Wavenumber (cm-1)

Assignment

Interaction

Intensity Change

1458

δas(CH3)

inter-chain

weak

1210

νas(C-O-C)+γas(CH3)

inter-chain

large

921

backbone stretching

intra-chain

medium

To quantitatively evaluate the effect of GONS loading on PLLA crystallization behavior, evolution of the normalized intensities of 921, 1458, and 1210 cm-1 for neat PLLA and its GONS nanocomposites is illustrated in Figure 4 according to Equation (2). Since the 921 cm-1 is assigned as the pure crystalline-specific band, its normalized intensity means the relative crystallinity. As shown in Figure 4a, the inclusion of 0.25 wt% GONSs results in the fastest crystallization rate, while the crystallization rate at a GONS content of 0.5 wt% is inferior to that of 0.25 wt% GONS/PLLA

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nanocomposite, but still superior to neat PLLA. A turning point for crystallization kinetics of PLLA nanocomposites from promotion to restriction occurs when adding 1.0 wt% GONSs. The further increase the content of GONSs to 2.0 wt% causes the lowest crystallization rate. These results are in good agreement with the DSC data. It is commonly accepted that the molecular chain conformational ordering is a necessary step for the formation of polymer crystals.17,21,45-47 The difference in crystallization behavior of neat PLLA and its GONS nanocomposites may pertain to the different conformational changes during isothermal crystallization. As displayed in Figure 4b-c, when adding 0.25 and 0.5 wt% GONSs, the intensity variations of the band at 1458 cm-1 present the same acceleration effect with that of the band at 921 cm-1. It can be seen that GONSs promote the occurrence of both inter-chain and intra-chain interactions. According our previous work,17 the acceleration effect is attributed to the CH-π interactions between PLLA chains and GONSs. While the addition of GONSs has little impact on the intensity change rates of 1210 cm-1. The reason for this possibly comes from the H-bonding interactions between the oxygen-containing functional groups on GONS layers and ester groups on PLLA molecular chains. Such interactions restrict the inter-chain conformational ordering about νas(C-O-C), which is not involved in the assignment of 1458 cm-1, counteracting the acceleration effect of CH-π interactions. As the GONS loading further rises up to 1.0 wt% and 2.0 wt%, the confinement effect is transformed into the dominant factor in suppressing the conformational ordering of δas(CH3) and νas(C-O-C) + γas(CH3), showing the same trend with that of the band at 921 cm-1. These results suggest that 21



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the crystallization mechanism of PLLA is inextricably linked with the influence of GONSs on the formation of conformational ordering.

Figure 4. Evolution of the normalized intensities of 921 (a), 1458 (b), and 1210 cm-1 (c) as a function of crystallization time during isothermal crystallization at 135 oC for neat PLLA and its GONS nanocomposites with different GONS loadings. 22


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DISCUSSION The above results depicted the panorama of PLLA crystallization behavior affected by various GONS loadings; that is, the crystallization kinetic of PLLA nanocomposites was initially accelerated and then hindered at the relatively high GONS loadings. On the basis of conventional crystallization viewpoint, the crystallization kinetic depends on by two stages: nucleation and growth. Recently, it is generally accepted that polymer crystallization is always accompanied with molecular conformational ordering, which is also relevant to the crystallization kinetics of polymer.17,47 Herein, we will elaborate the crystallization mechanism of PLLA nanocomposites in terms of nucleation, growth and conformational ordering. Nucleation is the precondition of crystallization, including homogeneous and heterogeneous nucleation. The heterogeneous nucleation can be effectively enhanced by adding heterogeneous nucleating agents. When adding 0.25 wt% and 0.5 wt% GONSs, the t0.5 values were respectively decreased from 3.89 min for neat PLLA to 3.17min and 3.73 min, indicating that GONSs are nucleating agents for PLLA crystallization. The acceleration effect could be further verified by the faster conformational ordering at 1458 cm-1 compared with neat PLLA due to the high aspect ratio of GONSs and their unique π-conjugated graphitic network (Figure 4b). In that case, PLLA molecular chains were apt to adsorb on the surface of GONSs via CH-π interactions, reducing the nucleating barrier and enhancing the crystallization kinetics. The underlying mechanism is so-called surface-induced conformational 23



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ordering as reported in our previous work.17 The lamellar crystal growth is currently related to the mobility and diffusion of polymer chains, which directly determines the ordered arrangement rate of polymer chains in the lattice. In this study, the crystallization kinetic of PLLA was restricted at a GONS loading of 1.0 wt%, wherein ~4.6% increase in t0.5 was obtained as compared with neat PLLA. And as the GONS loading went up to 2.0 wt% and 4.0 wt%, the t0.5 was respectively increased ~19% and ~74% relative to neat PLLA. The restriction effect was also confirmed in Figure 4b and 4c, in which the conformational ordering kinetics at 1210 cm-1 and 1458 cm-1 were slowed down by the relatively high GONS loadings. Since the nucleation effect of GONSs has been proved above, it can be speculated that the restricted crystallization rate and conformational ordering is attributed to the decline in mobility and diffusion of PLLA chains. In addition, the space for lamellar crystal growth is another important factor to affect the crystal growth rate. Recently, the confined crystal growth space is prevalent in the ultrathin polymer layer or film48 and polymer blends,49 in which polymer chains are remarkably inhibited and the growth of polymer lamellar crystals is greatly hampered. In the present GONS/PLLA nanocomposites, this effect possibly occurs at the relatively high GONS concentrations. In order to verify this, rheological measurement is performed to detect the changes of G' as a function of GONS concentrations. As shown in Figure 5, the neat PLLA exhibits characteristic response of a viscous polymer melt with a monotonous increase in G' as a function of sweeping frequencies, indicating a terminal flow behavior. For GONS/PLLA nanocomposites, no 24


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modification of G' is observed at high frequencies compared with neat PLLA, which suggests that the relaxation time of GONSs is much longer than that of the short-range dynamics of PLLA molecular chains. Nevertheless, upon increasing the GONS concentration, G' of PLLA nanocomposites gradually increases at the low frequencies. This result can be explained by the interactions between GONSs and PLLA chains, which slow down the motion of PLLA chains and restrain their relaxation. It is worth noting that when adding 1.0 wt% GONSs, G' seems to be independent on the sweeping frequency, reaching a plateau. This is indicative of a transition from liquid-like to pseudo-solid-like behavior. This non-terminal behavior can be attributed to the formed GONS network, which restricts the long-range diffusion and mobility of PLLA chains. The formation of network is often accompanied by a gelation behavior. Herein, the critical gelation GONS concentration can be estimated to be around 1.0 wt%. Such low critical gelation concentration of GONSs possibly results from their unique two-dimensional structure and large aspect ratio, as well as the interfacial adhesion between oxygen-containing functional groups of GONSs and the ester groups of PLLA molecular chains.28,50 What is even more impressive is that the non-terminal behavior of PLLA nanocomposites is quite remarkable at GONS concentrations of 2.0 wt% and 4.0 wt%, which implies formation of a more crowded and denser GONS network. In addition, owing to the viscoelastic behavior of polymer chains, the temperature dependence may be associated with polymer related network rather than nanofiller network.51 It is rational to speculate that GONS network is still stable existed in PLLA matrix when temperature decreases to the isothermal 25



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crystallization temperature. The GONS network can be used to effectively depict the crystalline circumstance in this study.

Figure 5. Frequency dependence of storage modulus (G') for neat PLLA and its nanocomposites at the indicated concentrations of GONSs. The enlarged is the region of low frequency.

In polymer nanocomposites, the attractive interactions between nanofiller and polymer matrix, without any doubt, frequently play a key role in the performance improvements. These interactions break the isotropic characteristic of original polymer, giving interfacial molecular chains some special features. In our work, in order to further quantitatively describe the GONS-built microstructure, a four-region model, including nanofiller region, interface region, constrained polymer region and unconstrained polymer region, is adopted.52 As illustrated in Figure 6, for brevity, GONSs are taken as rectangles with a thickness of t and a length of l,53 while d stands for the distance between GONS layers and s indicates the distance between successive GONSs on the same layer. The interface region is defined as binding polymer chains with nanofiller. The unconstrained polymer region is not affected significantly by 26


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nanofiller and invariably reserves the intrinsic characteristics of pristine polymer. The constrained polymer region is located between the interface region and the unconstrained polymer region, showing an elliptical shape,52,54 in which the expected range of this region extends about 50 ~ 100 nm away from the surface of nanofiller. And compared with pristine polymer, the constrained polymer always exhibits different performances, such as higher density, lower free volume and molecular chain mobility, owing to the strong interfacial adhesion between polymer matrix and nanofillers. Hence, it is deductive that the constrained polymer region plays a critical role in the course of polymer crystallization. For the purpose of further depicting the evolution of the constrained polymer region as a function of GONS concentrations, it is simply assumed that GONSs are fully exfoliated, homogeneously dispersed through the whole PLLA matrix and highly oriented to a certain direction.55,56 Note that GONSs are randomly dispersed in our PLLA nanocomposite samples shown in the Figure 1. Nevertheless, the “mean value” effect is still able to semi-quantitatively reflect the microstructure of PLLA nanocomposites. As shown in Figure 6a, the volume content (φ) of GONSs in the PLLA nanocomposites could be calculated as:

ϕ=

t ×l (l + s )(t + d )

(5)

Herein, assuming that the distance between GONS layers has the same value with that between successive GONSs on the same layer, namely d = s, theoretical GONS distances of PLLA nanocomposites can be calculated and the detailed results are listed in Table 3. The weight content is transformed to volume content by densities of PLLA matrix and GONSs, which can be taken as 1.25 g/cm3 and 1.80 g/cm3,29 respectively. 27



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Table

3.

Theoretical

distances

between

GONS

layers

Page 28 of 37

in

GONS/PLLA

nanocomposites. GONS loadings (wt%)

Volume content (%)

d (nm)

0

0

----

0.25

0.17

393.3

0.5

0.35

222.5

1.0

0.69

124.4

2.0

1.37

66.4

4.0

2.70

34.5

28


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Figure 6. Diagram of the four-region model and its evolution as a function of GONS concentrations.

When GONS concentration is lower than the critical gelation concentration as illustrated in Figure 6a, d is so large that there is little influence between the constrained polymer regions. GONSs with large specific surface area, serve as a template for PLLA chains to land on, promoting the conformational ordering kinetics (Figure 4b-c) and reducing the nucleating barrier. Nucleating effect of GONSs is dominant and mobility of PLLA chains is not signally restricted. Thus, the inclusion of GONSs results in an improvement on overall crystallization rate and PLLA crystals 29



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are apt to grow into spherulites, which is observed in DSC measurement (Figure 2a-c). Nevertheless, nucleating effect of GONSs almost saturates at a GONS loading of 0.25 wt% and further increasing GONS loading to 0.5 wt% gives rise to a decrease in facilitation effect on the overall crystallization rate. As GONS concentration rises to the critical gelation concentration, 1.0 wt% ca., the value of d is ~124.4 nm, about a factor of two relative to theoretical size of the constrained polymer region extended away from the surface of GONSs, so that the edges of the constrained polymer regions join together side by side (Figure 6b) and GONS network is formed in the PLLA matrix. This network verified by sold-like rheological behavior at low frequencies in rheological measurement (Figure 5) is able to divide PLLA matrix into many different small “separated zones”, which significantly hinder the interchange of PLLA chains. And owing to the universal existence of interactions between oxygen-containing functional groups on the GONS layers and PLLA molecular chains, H-bonding interactions shown in Figure 6b is widely existed between PLLA matrix and GONSs. Hence, the mobility and diffusion of PLLA molecular chains are remarkably suppressed, leading to a restricted conformational ordering kinetic at 1210 cm-1 and 1458 cm-1 (Figure 4b-c). The confinement effect on the diffusion of PLLA chains to the crystal growth fronts become a dominant factor in determining the overall crystallization kinetic, exhibiting a significant decrease in crystallization rate compared with neat PLLA. With further increasing GONS concentrations, higher than the critical gelation concentration, there are some parts of the constrained polymer region overlapping each other and a more crowded and denser GONS network is built 30


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in PLLA matrix. A more than 91 % reduction in d from 393.3 nm to 34.5 nm is obtained by adding 4.0 wt% GONSs, which provides a severely confined space for PLLA crystal growth. On this occasion, not only the mobility of PLLA chains, but also the space for PLLA crystal growth is dramatically declined, resulting in the heavily confined growth of PLLA lamellae between GONS layers (Figure 6c). This distinct two-dimensional crystal growth mode can be confirmed by the Avrami exponent n=2.2 shown in Table 1. A significant change of the four-region model occurs in Figure 6 with increasing GONS

concentrations,

constructing

different

crystalline

circumstance

and

consequently inducing the unusual crystallization behavior of PLLA matrix from promotion to restriction. This result further highlights the important role of crystalline circumstance in the process of polymer crystallization. And the semi-quantitative description of the crystalline circumstance provides a significant step forward to further understanding the crystallization behavior of polymer at a relatively high nanofiller loading.

CONCLUSION PLLA nanocomposites with different GONS loadings were prepared by solution coagulation method. The SEM images showed that GONSs are fully exfoliated and uniformly dispersed in the PLLA matrix. The effect of different GONS loadings on the crystallization behavior of PLLA nanocomposites was systematically studied via isothermal crystallization in the DSC and in situ FTIR measurements. In the PLLA 31



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crystallization course, GONSs could act as twofold effect on the PLLA crystallization kinetic. Firstly, GONSs could play a heterogeneous nucleating role to accelerate the crystallization rate. Secondly, GONSs could restrict the mobility and diffusion of PLLA chains and construct a constrained space for PLLA lamellae growth, resulting in a drastic decrease in crystallization kinetics. Below the critical gelation concentration, the conformational ordering kinetics was promoted via CH-π interaction and nucleation effect of GONSs was dominant to achieve an increased crystallization kinetic. However, with increasing GONS loading in the PLLA matrix, GONS network is built in PLLA matrix, which significantly constrained the mobility and diffusion of PLLA chains. The confinement effect became the dominant factor in determining the overall crystallization kinetic, giving rise to a remarkable reduction in crystallization rate. Especially, a relatively high GONS content with 4.0 wt% constructed a severely confined space with the overlapping constrained polymer regions, as well as more crowded and denser GONS network. The special structure resulted in a confined crystal growths of PLLA lamellae in a two-dimensional crystal growth mode. Thus, upon to GONS loading, an unusual crystallization behavior of PLLA from promotion to restriction was obtained. On the basis of exploring the effect of different GONS loadings on the PLLA crystallization behavior, the four-region model is an effective way to elaborate the mechanism of the confined crystallization behavior of polymer induced by the high concentration of nanofillers. In addition, the work presented here can provide favorable theoretical evidence to fabricate high performance and multi-functional polymer nanocomposites with high nanofiller 32


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loadings.

ACKNOWLEDGMENTS This work is supported by the National Outstanding Youth Foundation of China (Grant No. 50925311) and the National Natural Science Foundation of China (Grant No. 51120135002, 51121001).

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TABLE OF CONTENTS GRAPHIC

Poly (L-lactic acid) Crystallization in a Confined Space Containing Graphene Oxide Nanosheets

Hua-Dong Huang, Jia-Zhuang Xu, Ying Fan, Ling Xu, Zhong-Ming Li*

College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, P. R. China

*

Corresponding author. Tel.: +86-28-8540-6866; Fax: +86-28-8540-6866

E-mail address: [email protected] (Z.-M. Li)

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Poly(l-lactic acid) Crystallization in a Confined Space Containing

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