Epitaxial Crystallization of Isotactic Poly(methyl methacrylate) from

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B: Fluid Interfaces, Colloids, Polymers, Soft Matter, Surfactants, and Glassy Materials

Epitaxial Crystallization of Isotactic Poly(methyl methacrylate) from Different States on Highly Oriented PE Thin Film Zhixin Guo, Shuya Li, Xueying Liu, Jie Zhang, Huihui Li, Xiaoli Sun, Zhongjie Ren, and Shouke Yan J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b08193 • Publication Date (Web): 19 Sep 2018 Downloaded from http://pubs.acs.org on September 26, 2018

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The Journal of Physical Chemistry

Epitaxial Crystallization of Isotactic Poly(methyl methacrylate) from Different States on Highly Oriented PE Thin Film Zhixin Guo, † Shuya Li,† Xueying Liu, † Jie Zhang,†* Huihui Li,† Xiaoli Sun,† Zhongjie Ren,† Shouke Yan†§* † State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing, 100029, China §Key Laboratory of Rubber-Plastics, Qingdao University of Science & Technology, Qingdao 266042, China

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ABSTRACT: Through fixing the orientation of oriented PE thin film during meltrecrystallization with the help of a vacuum evaporated thin carbon layer, the isothermal meltand cold-crystallization of iPMMA on oriented PE substrate was studied. The results show that same parallel chain epitaxy of iPMMA on oriented PE substrate takes place in both cold- and melt-crystallization processes based on a two-dimensional lattice matching. However, the crystallization kinetics in the two processes is quite different. The induction time of iPMMA during melt-crystallization is significantly longer than during cold-crystallization (11 h vs 2.5 h). This is related to the different nucleation mechanism for cold- and melt-crystallization processes. On the other hand, the crystal growth rate of iPMMA from melt is much higher than from glassy solid state. This is associated to a higher molecular chain mobility of iPMMA in supercooled melt than in the frozen amorphous solid film.


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INTRODUCTION

Polymeric materials exhibit many advantages for advanced technology and therefore possess a permanent place of many sophisticated applications. This means that the properties of the polymeric materials can fulfill the requests for a wide range of specific uses. It is related to the fact that their properties can be well regulated by their multiscale structures in condensed state.1-5 For example, the stiffness and strength of highly oriented polymeric materials can exceed those of their isotropic counterparts by orders of magnitude and the electrical conductivity of doped and aligned conjugated macromolecules shows also significant increase compared with their non-oriented counterparts.1-4 For semicrystalline polymer materials, the specific crystal structure can even offer certain functionality of a polymer. As a representative example, the nonpolar αphase of poly(vinylidene fluoride) can be used only as common thermoplastics, while its β- and γ- counterparts are known polymers with strongest piezo- and pyro-electric activities.6 This leads to the precise control of multiscale structure of semicrystalline polymers to be an efficient way for improving their properties or even developing new functionality.

Up to date, many sophisticated studies have been performed to control some individual structures of polymers.7-16 It should be noted that a simultaneous and synergistic control of different structures for a given polymer remains, however, still a challenge. For this issue, surface-induced epitaxial crystallization is confirmed to be efficient for a simultaneous control of crystal modification and orientation.4,17-36 The epitaxial crystallization is generally realized by crystallizing a polymer on an oriented substrate. To this end, the epitaxy of a polymer with melting point lower than the used substrate is most easily achieved by melting the overgrowth polymer at temperature lower than the melting point of oriented substrate and subsequent


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recrystallization. For the polymers with higher melting point than the substrate, the epitaxial crystallization is frequently realized by cold-crystallization of it from amorphous glassy state on an oriented substrate. As examples, the epitaxial crystallization of poly(L-lactide) (PLLA) and isotactic poly(methylmethacrylate) (iPMMA) has been achieved through cold-crystallization of their amorphous thin film on oriented polyethylene (PE) and isotactic polypropylene substrates.37-39 It should be pointed out that the crystallization pathway, e.g., from melt or amorphous state, may influence the surface-induced epitaxy, including crystalline morphology and crystallization kinetics.40-42 However, a direct comparison on the cold- and melt-epitaxy of one polymer on certain substrate has not been reported. We report here a direct comparison of the cold- and melt-crystallization of iPMMA with Tm of ca. 160 °C on highly oriented PE substrate with Tm of about 130 °C. The melt-crystallization of iPMMA on oriented PE substrate was realized by keeping the molecular chain orientation of PE during melt-recrystallization and the iPMMA in supercooled molten state before the completion of PE crystallization. The fixing of molecular chain orientation of PE during melt-recrystallization has been achieved through vacuum evaporating a thin carbon layer onto the surface of highly oriented PE substrate film. The obtained results indicate that the epitaxial cold- and melt-crystallization of iPMMA on oriented PE substrate results in the same mutual chain orientation. However, the crystallization kinetics of melt- and cold-crystallization are quite different. This sheds light on the different crystallization mechanism of iPMMA regarding the nucleation and crystal growth processes.

EXPERIMENTAL SECTION

Materials.


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IPMMA material was purchased from Polymer Science, Inc., Canada. The tacticity of PMMA is 95% and its molecule weight is 3 x 105 g/mol. High-density polyethylene used in this work was obtained from Lanzhou Petrochemical, Chain. Its melting point is ca.135 °C as measured by DSC. The xylene and chloroform solvents were purchased from Beijing Chemical Reagent Co., Ltd. and used without further purification.

Sample Preparation.

Highly oriented PE thin films were prepared according to a melt-draw technique introduced by Petermann and Gohil.43 According their method, a small amount of a 0.5 wt% solution of the PE in xylene was poured and spread on a preheated glass plate uniformly, and the solvent was allowed to evaporate at the preparation temperature of ca. 130 °C. After evaporation of the solvent, the thin molten PE layer was then picked up by a motor-drive cylinder with a drawing speed of about 2 cm s-1. The thickness of the thus prepared film is around 50 nm (as measured by Dektak XT). For study the effect of oriented PE substrate on the crystallization of iPMMA, double-layered iPMMA/PE films for TEM and AFM study were prepared by spin-coating a 0.1 wt% iPMMA chloroform solution on highly oriented PE thin films, whereas those for FTIR study were prepared by solution-cast a 0.25 wt% iPMMA chloroform solution on the PE thin films. The layer thickness of the iPMMA was measured by Dektak Stylus Profilers and indicated at the corresponding places in text.

As confirmed in an early work,37 the spin-coated iPMMA layers are initially in amorphous state. This is ideal for conduct the comparison study of cold- and melt-crystallization of iPMMA on PE substrate. For study the crystallization of iPMMA on oriented PE substratefrom isotropic melt, the sample should be heated up to 200 °C for 5 min to ensure a complete melting of iPMMA. In


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this case, melting of the PE substrate film also takes place. To fixing the molecular chain orientation of PE substrate during the melt-recrystallization, a thin carbon layer was vacuum evaporated on one side of the PE thin film and then the iPMMA layer was spin-coated on the other side of PE oriented film, namely a sandwich structure of iPMMA/PE/C was fabricated. The thus prepared samples were heated up to and kept at 200 °C for 10 min and then cooling quickly down to and kept at 120 °C for 120 h. The cold-crystallization was simply realized by annealing the as-prepared iPMMA/PE double layer at 120 °C for 120 h. For a reasonable comparison, the PE substrate used for cold-crystallization has also been molten first at 200 °C and subsequently recrystallized at 120 °C to produce a more or less similar structure as that used for the meltrecrystallization.

Characterization.

For FTIR analysis, a Spectrum 100 FT-FTIR spectrometer (PerkinElmer) was used. Polarized FTIR was used to identify the mutual chain direction of iPMMA and PE. FTIR spectra in the wavenumber range from 450 to 4000 cm-1 were obtained by averaging 16 scans at 2 cm-1 resolution. For transmission electron microscopy (TEM) observation, a JEOL JEM-2100 TEM operated at 200 kV was used in this study. Phase contrast bright-field (BF) electron micrographs were obtained by defocus of the objective lens. In order to minimize radiation damage by the electron beam, focusing was carried out on an area; then the specimen film was translated to an adjacent undamaged area for recording the images immediately. AFM study was conducted with an Agilent Technologies 5500 operated under tapping model. Silicon tips with Al-coating, resonance frequency of 303 kHz and a spring constant of about 50 N/m were hired. The broadband dielectric measurement (DS, novocontrol GmbH) of iPMMA were tested at 120 °C


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for the samples heated from 0 °C up to 120°C and cooled from 200 °C down to 120 °C. The frequency range is 10-1-107 Hz. The sample was sandwiched between two copper electrodes (20 mm diameter).

RESULTS

Structure evolution of carbon-coated PE film before and after melt-recrystallization.

The structures of a carbon-coated PE substrate film before and after melt-recrystallization are by now familiar.44,45 For completeness of this paper, Figure 1 presents again the representative BF electron micrographs and corresponding electron diffraction patterns. Figure 1a reveals the lamellar structure of the as-prepared PE thin films. The PE lamellae are well aligned perpendicular to the drawing direction during film preparation, indicating the high orientation of melt-drawn PE thin films. The corresponding electron diffraction pattern (Figure 1b) indicates a fiber orientation of the molecular chains with the exposed lamellar edges are (hk0) lattice planes, i.e., the molecular chains (the crystallographic c-axis) arranged parallel to the drawing direction and crystallographic a- and b-axes rotated randomly about the chain direction. After melt at 200 °C and isothermally recrystallized at 120 °C, the BF electron micrograph (Figure 1c) shows still similar highly oriented lamellar structure. Also the electron diffraction pattern (Figure 1d) reveals a high degree of orientation. The electron diffraction pattern further confirms that, after melt-recrystallization at 120 °C, the carbon-coated PE films exhibit a double orientation with the crystallographic c- and b-axes oriented in the film plane, while the c-axis orientation is the same as the pristine films. Considering that the PE in the sandwiched iPMMA/PE/C sample has suffered a similar thermal treatment, the structure revealed in Figures 1c and d reflects the structure of PE served as the substrate for iPMMA during melt-crystallization.


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Figure 1. BF electron micrographs (a & c) and corresponding electron diffraction patterns (b & d) of carboncoated melt-drawn PE thin films before (a & b) and after (c & d) melt-recrystallization. The meltrecrystallization was performed by heating the sample up to 200 °C for 5 min and then cooling down to 120 °C for isothermal crystallization. The arrows in the BF images indicate the drawing direction during film preparation. The electron diffraction patterns are given in the same chain orientation as the BF images.

Cold- and melt-crystallization of iPMMA on highly oriented PE film.

As mentioned in the Introduction part, the cold-crystallization of iPMMA on oriented PE substrate has already been reported.37 Since the sample of iPMMA with isotacticity of 97% and molecular weight of 4.35 x 103 g/mol used in Ref. 37 is no longer available, both melt-and coldcrystallization of a new iPMMA material on the PE substrate were conducted here for a direct comparison. Figure 2 shows an AFM phase image and its corresponding electron diffraction pattern of the iPMMA cold-crystallized on an oriented PE substrate. The AFM image (Figure 2a) presents oriented iPMMA edge-on lamellar structure with the lamellae aligned perpendicular to the molecular chain direction of PE. This indicates the occurrence of epitaxial crystallization of iPMMA with its molecular chains parallel to the chain direction of the PE. The corresponding


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electron diffraction pattern shows diffraction spots of both PE and iPMMA, see Figure 2b, confirming the high orientation of iPMMA crystallized on oriented PE substrate. It should be mentioned that the diffraction pattern contributed by PE substrate in Figure 2b is similar to the one presented in Figure 1d and also has a close resemblance as the melt-crystallized one shown later in Figure 3b. The disappearance of (200)PE and intensity decrease of the (110)PE indicates a double orientation of it with b- and c-axes oriented preferentially in film plane. The appearance of strong (040) and (031) (which has been miss indexed as (421) in Ref. 37) diffraction spots of iPMMA suggests its bc lattice plane, namely the (100) lattice plane, in contact with the PE substrate. The alignment of (040)iPMMA along the (020)PE direction indicates a mutual parallel arrangement of the b-and c-axes of both iPMMA and PE, i.e., a parallel chain epitaxy as reported in Ref.37. This implies that the molecular weight of iPMMA does not influence the coldcrystallization behavior of it on oriented PE substrate.

Figure 2. An AFM phase image (a) and its corresponding electron diffraction pattern (b) of an iPMMA/PE/C layered sample, which has been annealed at 120 °C for 120 h. The arrows in the pictures indicate the molecular chain direction of PE substrate.

Figure 3 shows the AFM phase image and its corresponding electron diffraction pattern of the iPMMA melt-crystallized on an oriented PE substrate. The AFM image shown in Figure 3a reveals a similar oriented iPMMA edge-on lamellar structure as that shown in Figure 2a,


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indicating a similar epitaxial crystallization of iPMMA with its molecular chains parallel to the chain direction of the PE. The corresponding electron diffraction pattern shown in Figure 3b is also essentially the same as Figure 2b, confirming the high orientation of iPMMA meltcrystallized on oriented PE substrate. From this point of view, it can be concluded that (i) the crystallization of iPMMA does not start before completion of PE oriented recrystallization, otherwise the PE will not affect the crystallization of iPMMA; and (ii) the early crystallized oriented PE induces the oriented crystallization of iPMMA from melt, i.e. the occurrence of epitaxy.

Figure 3. An AFM phase image (a) and its corresponding electron diffraction pattern (b) of an iPMMA/PE/C layered sample, which has been heat-treated at 200 °C for 5 min and then cooled down to and kept at 120 °C for 120 h. The arrows in the pictures indicate the molecular chain direction of PE substrate.

Cold- and melt-crystallization kinetics of iPMMA on highly oriented PE film.

The above morphological study shows that the initial state does not affect the mutual orientation of iPMMA/PE epitaxial crystallization. We now turn to the analysis of crystallization kinetics of iPMMA on the PE substrate, since both morphology and crystallization kinetics are of fundamental interest for the understanding of polymer crystallization. The crystallization kinetics of iPMMA has to our knowledge not been paid much attention since the crystallization of it is


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extremely time consuming. The only study on the melt-crystallization kinetics of iPMMA has been performed by de Boer et al.46 They found that the iPMMA has a maximum growth rate of ca. 1x10-3 µm/min at 120 °C. It is about the 1/300 maximum growth rate of isotactic polystyrene.47 On the other hand, surface-induced cold-crystallization kinetics of iPMMA was reported in the literature.37,48 While Brinkhuis and coworkers studied the cold-crystallization kinetics of iPMMA induced by an oriented Langmuir-Blodgett thin film of itself,48 we have reported the cold-crystallization kinetics of it on an oriented PE substrate.37 The meltcrystallization kinetics of iPMMA could not be conducted in those cases since the substrate will also be molten during heat-treatment. With the fixing effect of vacuum evaporated carbon layer on oriented PE substrate, the melt-crystallization kinetics of iPMMA on oriented PE substrate can be realized. The point here is to compare the crystallization kinetics of iPMMA on the oriented PE substrate from different initial states. This has been done by FTIR experiments. To perform a detailed IR analysis, the band assignments reported in the literatures were used.48-51 The band at 1295 cm-1 has been assigned to the characteristic band of the iPMMA crystalline phase. Therefore, it has been used to analyze the crystallization kinetics of iPMMA.

Figure 4. Time-dependent IR spectra (top panel) and their corresponding difference spectra (bottom panel) of the iPMMA/PE/C samples collected during the isothermal (a) cold- and (b) melt-crystallization processes at 120 °C in the range of 1300-900 cm-1. The difference spectra were obtained by


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subtracting the spectrum at the time when the samples were just heated up or cooled down to 120 °C from those obtained after annealing at 120 °C for different times.

Figure 4 shows the time-dependent IR spectra collected during cold- and melt-crystallization at 120 °C and the corresponding difference spectra (which were obtained by subtracting the spectrum at the time when the samples were just heated up or cooled down to 120 °C from those obtained after annealing at 120 °C for different times) of an iPMMA on oriented PE film. The spectra in the range of 1300-900 cm-1 was selected since the PE shows no absorption peaks.52,53 From the time-dependent IR spectra (top panels of Figures 4a and b), it can be found that, in both cold- and melt-crystallization processes, the intensities of some absorption bands get increased while some of them become weaker with time. This can be more clearly observed in the difference spectra shown in the bottom panels of Figures 4a and b. The time-dependent intensity increment of band at 1295 cm-1 verifies undoubtedly the occurrence of iPMMA crystallization. During the crystallization process, the intensities of the bands at 1261, 1233, 1167 and 1132 cm-1 decrease with time. They must be amorphous related. On the other hand, intensity increases of the bands at 1250 and 1149 cm-1 are also observed. This implies that these bands are also crystalline related. Therefore, the intensity variation of the 1295, 1250 and 1149 cm-1 bands can be used to analyze the crystallization kinetics of the iPMMA on oriented PE substrate. To this end, the Avrami equation, which describes the development of relative degree of crystallinity (Xt) with crystallization time (t), is usually employed to analyze the isothermal crystallization kinetics quantitatively. The normalized peak height of the crystallization sensitive bands at 1295 and 1250 cm-1 as a function of crystallization time and the generated Avrami plots from the data of the 1295 cm-1 bands for cold- and melt-crystallization processes are presented in Figure 5. From Figures 5a and c, the half-crystallization times (t1/2) were calculated to be 32 and 38 h for the cold- and melt-crystallization, respectively. It seems that the cold-crystallization of iPMMA is


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faster than its melt-crystallization. This is actually caused by the difference in film thickness. Considering that the epitaxial crystallization of iPMMA starts from the PE surface and propagates into the bulk layer, longer time is required for a fully crystallization of a thicker film. The thicknesses of the iPMMA layers used for cold- and melt-crystallization were measured to be 340 and 450 nm, respectively. In this context, the propagation rate of iPMMA from the interface to the bulk can be estimated simply by the ratio of sample thickness and two times the t1/2. The obtained values are 5.3 nm/h for cold-crystallization and 5.9 nm/h for meltcrystallization. This demonstrates that the melt-crystallization of iPMMA is faster than its coldcrystallization. The k values obtained from Figures 5c and d are 12.34 for cold-crystallization and 15.7 for melt-crystallization, also indicates a faster melt-crystallization with respect to the cold-crystallization.

Figure 5. The normalized peak height of the crystallization bands at 1295 and 1250 cm-1 as function of time during (a) cold- and (c) melt-crystallization of iPMMA on oriented PE substrate. At, A0, and A∞ present the intensity of the band in the IR spectra at time t, at the beginning of isothermal crystallization and after 120 h at 120 °C. The generated Avrami plots from the data of the band at 1295 cm-1 for (b) cold- and (d) melt-crystallization processes.


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As well established, the Avrami exponent n is characteristic of the nucleation type and the crystal growth geometry. For heterogeneous nucleation, while a value of ~3 is usually adopted for bulk crystallization, a value of ~2 will be obtained for the growth in two dimensional thin film. In the present case, the calculated Avrami exponent (n) of 1.6 and 1.9 for cold- and melt-crystallization, respectively. This reveals a two dimensional thin film crystallization of iPMMA triggered by PE substrate through heterogeneous nucleation.

Another thing should be mentioned here is about the onset crystallization of iPMMA from different states. From Figure 5a, an induction period of approximately 2.5 h was observed for the cold-crystallization. It is much longer than that reported in Ref. 37, where the cold-crystallization of iPMMA on PE substrate starts ca. 25 min after heated to 120 °C. This should be caused by the different molecular weight. From Figure 5c, we can find that the induction time for the meltcrystallization process is ca. 11 h, which is significantly longer than that of the coldcrystallization process. This is somewhat hard to be understood when the same heterogeneous nucleation process is considered and will be discussed in the following section.

DISCUSSIONS

According to the above experimental results, several aspects should be discussed here. First, the obtained experimental results clearly indicate that the epitaxy of iPMMA on oriented PE substrate with the same mutual orientation can be realized by both cold- and melt-crystallization. This is nothing surprising when geometric matching is considered as the origin of polymer epitaxy. For the iPMMA and PE system, a matching is found between interplane distances of (010)PE (0.494 nm) and (050)iPMMA (0.488 nm) with a mismatching of ca. 1.4%. Actually, another matching along the chain axes of both polymers with a discrepancy of ca. 3.5% is also


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well within the upper limit of 15% for the occurrence of polymer epitaxy.54 This implies that the epitaxial crystallization of iPMMA on highly oriented PE film, both by cold- and meltcrystallization, is based on the same two dimensional lattice matching.

Second, the variation of induction period in different crystallization processes can be understood in the following way. Differences in crystallization kinetics from melt and cold crystallization have been frequently reported. For this issue, the crystallization of Poly(L-lactic acid) (PLLA) by quenching from the melt or by heating from the glassy state has been well studied.40,41,55-58 It was found that the crystallization of PLLA from glassy state is much fast than from melt state. This has been correlated to the formation of crystalline nuclei during the quenching of the melt, which is believed to be difficult to avoid. Moreover, very recently, the research group of ThurnAlbrecht has demonstrated that surface-induced crystallization on a solid substrate can occur through a frozen crystalline layer, which is stable above the bulk melting temperature.59, 60 Upon cooling, this layer acts as an “ideal crystal nucleus” and significantly accelerates the nucleation process. In a previous study of our group on the epitaxial cold-crystallization of PLLA on oriented PE substrate, it was also confirmed that there exists an oriented thin layer in the amorphous PLLA film prepared by spin-coating PLLA solution onto highly oriented PE film.38 This oriented layer can easily promote the epitaxial cold-crystallization of PLLA on the PE substrate. Turn back to the present case, an oriented thin amorphous iPMMA layer is also expected in the as-prepared iPMMA/PE sample. To verify this hypothesis, polarized FTIR measurements on the as-prepared iPMMA/PE sample before and after heat-treatment were conducted. Figure 6 shows the polarized FTIR spectra of an iPMMA/PE/C sample before and after annealing at 120 °C for 120 h. The electron vectors were set perpendicular (90°) and parallel (0°) to the molecular chain direction of PE substrate, respectively. Differences between


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the polarized FTIR spectra with the electron vector perpendicular and parallel to the molecular chain direction of the PE thin film can be recognized. From Figure 6a, it is clear that the absorption intensities of 2918 and 2850 cm-1 bands, assigned to the asymmetric and symmetric stretching vibration of CH2 groups of PE having a perpendicular transition moment with respect to the main chains, are much higher in perpendicular polarization than in parallel polarization. This illustrates the high orientation of the substrate PE molecules. On the other hand, the crystallization sensitive bands of iPMMA at 1734, 1295 and 1250 cm-1, which have also a perpendicular transition moment to the main chains, exhibit a similar intensity dependence of polarization. This again confirms a parallel chain alignment of iPMMA with PE after epitaxial crystallization. We here pay attention to the as-prepared iPMMA/PE/C sample. As presented in the left part of Figure 6b, the amorphous sensitive band at 1732 cm-1 rather than the crystalline sensitive 1734 cm-1 band was observed in these FTIR spectra, reflecting the amorphous feature of the iPMMA. However, with close inspection, the intensity difference of this band with perpendicular and parallel polarization can be identified. This implies that some iPMMA chains in amorphous state are oriented. The oriented iPMMA layer is clearly induced by the oriented PE substrate and should be very thin (most likely a monolayer). Therefore, the intensity difference of this band with perpendicular and parallel polarization is very small. However, the contribution of this thin layer to the FTIR spectra should be increased when the overall sample thickness is reduced. This has really been observed in right part of Figure 6b with reduced iPMMA sample thickness. These oriented iPMMA molecular chains or chain segments can transfer to crystalline nuclei and speed up the nucleation of iPMMA significantly. Thus, a shorter induction period is needed for the cold-crystallization. By contrast, during the melt-crystallization, the sample was first heated up to 200 °C for 5 min. This leads to a fully relaxation of the iPMMA molecular


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chains, as also observed for the frozen layers of PE and polycaprolactone on the graphite surface by Thurn-Albrecht and coworkers.59-60 Consequently, upon cooling down to 120 °C for isothermal crystallization, the nucleation of iPMMA takes place in the random-coiled supercooled melt, which happens after a much longer time with respect to the cold-crystallization process.

Figure 6. Polarized IR spectra of iPMMA/PE/C layered samples after (a) and before (b) thermal treatment at 120 °C for 120 h. 0° and 90° in the pictures indicate the electron vector parallel and perpendicular to the molecular chain direction of PE substrate film, respectively. The thickness of the iPMMA layers were measured to be 400 nm for (a), 800 nm for left part of (b) and 50 nm for right part of (b).

Third, combined with the much longer induction time and a faster crystallization process of iPMMA from melt than from amorphous film, it is concluded that the crystal growth of iPMMA from supercooled melt is much faster than from the solid amorphous state. This may be explained in the follow way. During crystal growth, diffusion of the non-crystallizable component away from and chain segment in the amorphous region to the growth front is necessary. For the cold-crystallization process, the molecular chains are originally in a frozen state. On the other hand, the melt-crystallization process was realized by heating the materials up to 200 °C and then cooling down to 120 °C. It is still in a supercooled molten state. In this case, the chain mobility of the iPMMA might be higher than that in the frozen amorphous state during


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cold-crystallization process. In order to confirm the result obtained from the in-suit FTIR, supplementary Broadband Dielectric Spectroscopy (BSD) investigation was conducted. Dielectric loss and dielectric constant reflect the motor ability of the molecular chain. The greater of the dielectric loss and dielectric constant, the higher the motion ability of the polymer chains. As presented in Figures 7a and b, the dielectric loss and dielectric constant for the iPMMA sample cooled down from melt state to 120 °C are larger than the sample heated from glassy state at room temperature up to 120 °C. This indicates a higher molecular chain or chain segment mobility of the former than the latter case, which may be caused by the increase of the density of the fluctuating dipoles. Furthermore, there exist both α-relaxation and β-relaxation peak in BSD for the sample heated to 120 °C from glassy state, with the peaks locating at 900Hz and 1.4*106HZ. As for the PMMA sample cooled down from melt state to 120 °C , the α- and βrelaxation merge into one relaxation peak.61 They locate at around 106 Hz. For long chain polymers, the α peak represents the motion of the molecular chain segments.62 The shift of αrelaxation to higher frequencies suggests the enhancement of the mobility of the molecular chain segments. Therefore, the present results reflect the importance of chain mobility in polymer crystallization.

25 to 120 200 to 120

5.5 5.0 4.5

Dielectric constant

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4.0 3.5 3.0 2.5 2.0

a

1.5 1

10

100

1000

10000 1000001000000 1E7

1E8

Frequency(Hz)


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Figure 7. The dielectric constant (a) and dielectric loss (b) for the iPMMA samples heated from amorphous glassy state at room temperature up to 120 °C (squares) and cooled from 200 °C down to 120 °C (circles).

CONCLUSIONS

By fixing the molecular chain orientation of PE during melt-recrystallization process via a vacuum evaporated thin carbon layer, the melt-crystallization of iPMMA on highly oriented PE substrate has been studied and compared with its cold-crystallization process. Through the obtained results, following conclusions have been made:

(i)

Epitaxial crystallization of iPMMA on the oriented PE thin film can be achieved through

both cold- and melt-crystallization with exactly the same common (100) contact planes of both polymers and mutual chain parallel orientation. This is reasonable since they are based on the same two-dimensional lattice matching.

(ii)

The induction time of iPMMA melt-crystallized on the PE substrate is much longer than

that of cold-crystallization process. This is due to the formation of some ordered iPMMA chain segments in amorphous state on the PE substrate during sample preparation. It is these ordered chain segments that serve as pre-nuclei and initiate an early nucleation. On the contrary, during melt-crystallization, by heating the sample up to 200 °C results in a complete relaxation of the ordered iPMMA chain segments. Therefore, a time-consuming nucleation process is found for the melt-crystallization.


19


(iii)

Page 20 of 30

Even though the nucleation of iPMMA from melt takes a much longer time, the growth

of the crystals is much faster from melt than from amorphous solid film. This originates from a higher chain mobility of iPMMA in the supercooled melt than in the glassy solid state, as confirmed by BSD measurements.

ACKNOWLEDGEMENTS

The financial support of the National Natural Science Foundations of China (No. 21434002 and 51521062) is gratefully acknowledged. AUTHOR INFORMATION Corresponding Author *Jie Zhang, E-mail: [email protected], Tel: +86-10-64455928 Shouke Yan, E-mail: [email protected], Tel: +86-10-64455928 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. REFERENCES (1) Smith, P.; Lemstra, P. Ultra-high-strength Polyethylene Filaments by Solution Spinning/Drawing. J. Mater. Sci.1980, 15, 505-514. (2) Dong, H; Jiang, S; Jiang, L; Liu, Y; Li, H; Hu, W; Wang, E; Yan, S; Wei, Z; Xu, Z; Gong; X. Single Crystalline Nanowires of a Rigid Rod Conjugated Polymer. J. Am. Chem. Soc. 2009, 131, 17315-17320.


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Table of Contents Graphic


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Epitaxial Crystallization of Isotactic Poly(methyl methacrylate) from

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