Effective enhancement of poly(bisphenol-A carbonate) crystallinity with polyamide-66 ionized nucleation through ion-dipole interactions coupled with plasticization

To improve the crystallinity of poly(bisphenol-A carbonate) (PC). Polyamide-66 (PA-66) is ionized by CaCl2 to prepare the various degrees of ionization (DIs) of PA-66 ionenes (CaCl2–PA-66) as nucleating agents to co-modify PC with cholesterol nonanoate (CN) as plasticizers. Compared with non-ionized PA-66, CaCl2–PA-66 enhance the crystallinity of PC. Concretely, with the DI of CaCl2–PA-66 is raised from 0 to 37.2 mol%, the crystallinity of PC/CN/CaCl2–PA-66 (90/5/5 w/w/w) is first increased from 19.2% to 31.5% and then decreased to 16.9%. This is attributed to the stronger ion-dipole interactions between the ionized nucleating agents and PC, which enhance the compatibility and further dispersibility to create finer crystal with a denser distribution, and the nucleation efficiency is elevated thus facilitating the PC crystallization. However, when the DI of CaCl2–PA-66 is overly high (≥11.4 mol%), it leads to excessive ionic crosslinking, which reduces dispersibility and nucleation efficiency. Therefore, the modification of PC with a suitable DI of CaCl2–PA-66 can advance its crystallinity, which is a novel and effective approach.


Introduction
Poly(bisphenol-A carbonate) (PC) is widely used in optical discs, optical lenses, automobiles, construction, and industrial machinery etc, due to its high mechanical property, great dimensional stability, low creep, excellent heat and cold resistance, and superior light-transmission [1,2]. PC is an amorphous polymer because the presence of two phenyl groups in a repeating structure unit, which makes the molecular chain of PC very rigid and difficult to crystallize. However, PC, as a typical condensate, has a regular molecular chain structure that without no bonding, geometric and rotational isomerism, and thus high crystallization potential. Therefore, to expand the applications of PC, it is expected to promote the crystallinity to further enhance its mechanical properties, thermostability, and chemical resistance. The crystallization of polymers is generally divided into two processes: nucleation and crystal growth. Due to the rigid molecular chains, the crystallization kinetic process of PC is very slow, and the crystal growth rate of PC is maximum at 190°C [3]. However, even at this temperature, PC still needs more than 100 h to form the first nucleus, at which the semi-crystallization time (t 1/2 ) of PC is 12.5 days [3][4][5]. Therefore, the improvement of the crystallization property of PC should start from the nucleation and crystal growth at the same time. At present, there are various options for crystallization modification of polymers, such as the nucleating agent, plasticizer, and nucleating agent-plasticizer coupling modification.
2.3. Melt blending of PC, CaCl 2 -ionized PA-66, and CN Dried PC, CN and CaCl 2 -PA-66 with different degrees of ionization (DIs) were mixed in the ratio of 90/5/5 w/ w/w. A twin-screw micro mixer (HAAKE Mini Lab II) was preheated to 270°C, and the mixture was subsequently added to it and mixed at 300 rpm for 3 min. The mixing process was carried out under a N 2 atmosphere. After melt blending, the material was extruded, marked, and stored.

Inductively coupled plasma optical emission spectrometry
An inductively coupled plasma optical emission spectrometer (ICP-OES, Agilent 730) was used to investigate the ionic content of CaCl 2 -PA-66. The samples of a certain mass were weighed into Teflon ablation tubes separately. HCl (1 ml), HF (1 ml) and H 3 NO 3 (5 ml) were added to each tube, and the tubes were shaken thoroughly to mix well. The curve of the standard solution was measured before testing, and subsequently the sample mass and volume were entered. Afterward, the well-dissolved solution was tested in turn, the dilution beyond the curve range was tested again, and the concentration of elemental Ca in the sample was determined by the calibration curve of the standard test solution. Finally, the actual ionic content of the sample was further calculated.

Differential scanning calorimetry
All samples were tested according to the standard of ISO 11357 using a differential scanning calorimeter (DSC) (PerkinElmer, DSC8000). Before the test, the DSC was calibrated for temperature and heat flow using indium specimens, and two empty crucibles were scanned using the same procedure of test to obtain a baseline. For the runs, ∼5 mg of the sample was encapsulated in an aluminum crucible and subsequently placed in a chamber. PA-66 and CaCl 2 -PA-66 were first heated from 30 to 275°C to eliminate the preparation heat history, then cooled down to 30°C to obtain the cooling curve, and finally heated from 30 to 275°C to collect the heating curve. Afterward, PC/CN, PC/PA-66, and PC/CN/CaCl 2 -PA-66 were first heated from 30 to 275°C to eliminate the preparative thermal history, and subsequently cooled to 190°C for 24 h. At the end, they were cooled to 30°C and then heated again to 275°C to collect the second heating curve. The experiments were conducted under a N 2 atmosphere with a temperature sweep rate of 10°C min −1 .

Polarized optical microscopy
The crystal morphology of PC/CN/CaCl 2 -PA-66 was observed by a polarizing optical microscope (Leica, DM2500P). A clean slide was placed in the center of the hot table, and a sample (∼2 mg) was placed in the center of the slide. For the runs, the hot table was heated at a heating rate of 10°C min −1 from 30 to 275°C, and held for 1 min at 275°C to eliminate the thermal history. During the holding period, after the sample was completely melted, another slide was taken with forceps to cover the sample, and the slide was gently pressed to stretch the sample completely to obtain a slide-sample-slide sandwich structure with a polymer film in the center. Then, the table was cooled down to 190°C, and followed transferred to a vacuum drying oven at 190°C for 24 h to allow the sample to fully crystallize. After insulation, the sample was allowed to cool naturally to RT, and subsequently transferred to a polarized hot table sample bath. The crystal structure was observed at RT by selecting a suitable eyepiece and objective magnification, followed by a temperature rising to 275°C at a rate of 10°C min −1 , during which the gradual melting of the sample crystals was observed and photographed.

Wide-angle x-ray diffraction
A polyimide circular film with a diameter of ∼10 mm, which works as a carrier to bear the sample, was placed in a sample cell on the hot stage of the polarizing microscope. The lumpy sample (∼5 mg) was placed on the film, and the hot stage was set to heat up to 275°C at a rate of 10°C min −1 and held for 1 min. After the sample was completely melted, another piece of polyimide film was placed on the sample and lightly pressed to stretch the sample into a membrane, which was then cooled down to 190°C at a rate of 10°C min −1 and subsequently transferred to a vacuum oven at 190°C for 24 h to annealing. After annealing, the sample was cooled to RT and the polyimide film was removed to obtain a membraniform sample (∼50-60 μm), which was placed on the sample stage of an Empyrean-type x-ray diffractometer (graphite monochromator, Cu Kα radiation at 1.5406 Å). Then, the sample tested in reflection mode for scanning with a speed and range (2θ) of, respectively, 4°min −1 and 10°-50°, and the voltage and current were 45 kV and 40 mA.

Scanning electron microscope
To observe the dispersion effect of PA-66 and CaCl 2 -PA-66 with different ionization degrees in the PC system, the modified PC samples were treated by etching method, and a scanning electron microscopy (SEM) (Zeiss Sigma 500) was used to observe the pores left by the nucleating agent after etching and to analyze them. The sample preparation method was referred in section 2.7 to obtain modified PC sample membranes, which were subsequently immersed in formic acid solution for 48 h at RT to fully dissolve the nucleating agent on the surface. After etching, the samples were washed in distilled water to remove residual formic acid, subsequently dried overnight in a fume hood, and followed by vacuum drying at 60°C for at least 24 h. Afterward, the samples were gold sprayed, observed and photographed at a voltage of 3.0 kV using the SEM.

Results and discussion
3.1. Evidence of success in ionizing PA-66 Figure 1 shows the (a) cooling curves and (b) second heating curves of PA-66 (curve 1) and 5 wt%, 10 wt%, 20 wt%, and 50 wt% CaCl 2 -PA-66 (curves 2-5) in DSC. As shown in curve 1 of figure 1(a), the crystallization temperature (T m,c ) of PA-66 is 234.2°C. As the degree of ionization (DI) increases, the cold crystallization peaks of curves 2 and 3 both move to low temperature and the T m,c decreases to 196.8°C and 180.7°C, respectively. Simultaneously, as the DI continues to increase, 20 wt% and 50 wt% CaCl 2 -PA-66 cold crystallization peaks are no longer detectable and hence the crystallization ability of the polymers is almost lost. Besides, the gradual increase of the cold crystallization peak-width is also proved the change of the crystallization ability of the polymer after ionization, because the half-peak width of the cold crystallization peak is related to the grain size distribution of the polymer crystals, the higher the half-peak width, the wider the size distribution. In addition, table 1 shows that the ΔH m,c also decreases monotonically from 72.3 to 52.1 and 9.2 J g −1 for the first three groups of specimens. Together, the above phenomena confirm that the crystallization ability of PA-66 is weakened and the degree of crystal perfection is reduced with increasing CaCl 2 content. Figure 1(b) shows the second heating curves. As shown in curve 1, PA-66 exhibits melt bimodal peaks that the melting temperature (T m ) of 260.2 and 250.8°C. After ionization, the melt bimodal peaks are changed to unimodal melt peaks. Meanwhile, the T m is decreased and the half-peak width of melt peak is increased with the increase of the DI. The ΔH m (table 1) is also decreased from 78.3 to 41.7 and 32.6 J g −1 . The melting peak is Composition ( where M 1 is the molar mass of the structural repeating unit of PA-66, 226.36 g mol −1 ; M 3 is the molar mass of the structural repeating unit of PA-66 after ionization with CaCl 2 , 337.34 g mol −1 ; M Ca is the relative atomic mass of element Ca, 40.08 g mol −1 ; c Ca is the mass of element Ca per gram of sample in g g −1 . Assuming that the degree of reaction of the solution neutralization experiment is 100%, the theoretical ionization content of the five DIs of CaCl 2 -PA-66 can be calculated by equation (1) as 0, 10.2, 20.3, 40.7, and 101.6 mol%, respectively. However, the ionization reaction is realistically carried out incompletely, and the actual DIs of PA-66 and the four ionized products can be calculated as 0.028, 6.5, 11.4, 19.3 and 37.2 mol%, respectively, based on the ICP-OES results. Due to the products are purified several times, the free CaCl 2 is completely removed from the system, the measured Ca content can be regarded as the ionization grafting, and the actual DI of the CaCl 2 -PA-66 is increased more than three orders of magnitude compared with pure PA-66. It proved that the ionization reaction is successful and the reaction degree is at a high level. Moreover, the DI is increased with the rase of CaCl 2 content and showed a corresponding gradient change, which is in accordance with the experimental expectation.
3.2. CN Increases the chain mobility and thus crystallinity of PC CN, as a plasticizer that can enhance the movement of polymer molecular chains, is used to promote PC crystallization. Curves 1 and 2 in figure 2 demonstrate the second DSC heating curves of PC and PC/CN (95/5 w/w), respectively. The two curves indicate that the glass transition temperature (T g ) of PC is decreased from 148.8 to 129.0°C after adding 5 wt% of CN, and CN played a better plasticizing effect and effectively enhanced the motion of PC molecular chains. Curve 3 shows the second heating curve of PC/CN after holding at 190°C for 24 h. In addition to the glass transition step at 126.9°C, curve 3 suggests a new melt bimodal peak at 190.9°C and 223.1°C, which is attributed to the melt crystallization peak of PC. This proves that under the CN plasticization, PC can crystallize significantly after annealing at 190°C for 24 h. Meanwhile, due to the reduction of amorphous components caused by PC crystallization, the crystals produce partial physical crosslinking, which makes the glass transition of the system less obvious. Furthermore, the melting enthalpy (ΔH m ) of this molten bimodal peak is 14.4 J g −1 , and according to the equation (2), the crystallinity of PC/CN can reach to 13.8% after 24 h of holding and crystallization.
where X c is the crystallinity of PC, ΔH m is the melting enthalpy of PC in J g −1 , ΔH m,0 is the melting enthalpy when the crystallinity of PC is 100%, ΔH m,0 = 109.7 J g −1 [7]; w is the mass fraction of PC in the modified system.   Table 3 exhibits that the crystallinity of the coupled modified system PC/CN/PA-66 is 19.2%, which is higher than the 13.8% of the PC/CN system. This can confirm that the nucleating agent plays a certain heterogeneous nucleation role to enhance the crystallinity of PC under the presence of CN as a plasticizer. The ΔH m and crystallinity of PC modified with 6.5 mol% CaCl 2 -PA-66 nucleating agent were increased to 27.8 J g −1 and 28.2%, respectively, which proves that the crystallinity of PC is further improved. This indicates that the nucleating agent PA-66 is more effective in inducing heterogeneous nucleation after ionization, and can enhance the crystallization performance of PC more effectively. As the DI of nucleating agent increased, the melting point of PC is remained basically, but the crystallinity shows a trend of rising and then falling, reaching the highest at the DI of nucleating agent of 11.4 mol%, when the ΔH m and crystallinity of PC are, respectively, 31.1 J g −1 and  31.5%. After which, as the DI of nucleating agent is continued to rise, the crystallinity is gradually reduced, and at the DI of 37.2 mol%, the crystallinity is decreased significantly to 16.9%, which is even lower than that of the unionized modified PA-66 nucleating agent.
The results confirm that the promotion effect of nucleating agent ionization on PC crystallization is not monotonically elevated, but has a side effect on the crystallization of the system after reaching a certain limit, thus inhibiting the crystallization of PC. With the introduction of ion-dipole interactions, the compatibility between PA-66 and PC is improved, the nucleating agent is more uniformly dispersed, the grain size is smaller, and the number density is increased, which is beneficial to both heterogeneous nucleation and nucleation rate, both of which can enhance the crystallization performance of PC more effectively. However, with the increase of nucleating agent ionization, the ion-dipole interactions in the system are stronger, the nucleating agent is more finely dispersed, and the amorphous interface layer volume is increasing, which hinders the crystallization of PC [45]. Furthermore, too much ionization of nucleating agent will increase its ionic crosslinking density, the viscosity of nucleating agent will become more difficult to disperse, and the number density of dispersed phase will decrease, which is not conducive to heterogeneous nucleation. From the above analysis, it can be seen that CaCl 2 -PA-66 with too high ionization degree can't effectively enhance the PC crystallization performance instead, and the detailed mechanism will be analyzed in detail in section 3.4.
The crystalline morphology of PC/CN/CaCl 2 -PA-66 after annealing at 190°C for 24 h and the melting process at a heating rate of 10°C min −1 are shown in figure 4. The bright spots in figure 4 represent the anisotropic crystalline fraction of the PC, while the dark areas represent the amorphous fraction. The comparison of crystallinity at RT in the first row shows that the bright zone in the polarized photograph of PC/ CN/11.4 mol% CaCl 2 -PA-66 is uniformly distributed and occupies the largest proportion, and the crystalline zones of PC/CN/0-37.2 mol% CaCl 2 -PA-66 show a trend of increasing and then decreasing, respectively, which is in accordance with table 3. In addition, the size distribution of grains in the bright crystalline region of PC/CN/11.4 mol% CaCl 2 -PA-66 is finer than that of other groups, while the percentage of bright regions of PC/CN/37.2 mol% CaCl 2 -PA-66 is significantly lower, which also confirms that the ion-dipole interactions introduced by ionic nucleating agents play a significant role in promoting the dispersion of nucleating agents, but the excessive ion-dipole interactions inhibit the dispersion of nucleating agents and reduce the number of PC crystal nucleus [45]. As the temperature increases, the bright areas of PC/CN/PA-66 and PC/CN/37.2 mol% CaCl 2 -PA-66 were the first to begin to decrease significantly, indicating that their crystal areas were the first to begin to melt. As the temperature continued increase 235°C, the bright areas of PC/CN/PA-66, PC/ CN/19.3 mol% CaCl 2 -PA-66, and PC/CN/37.2 mol% CaCl 2 -PA-66 are completely disappeared at 235°C. Moreover, PC/CN/6.5 mol% CaCl 2 -PA-66 and PC/CN/11.4 mol% CaCl 2 -PA-66 are completely melted at 245 and 255°C, respectively. This trend may be attributed to the higher crystallinity of these, which proves more perfect crystals and thus required a higher temperature for complete melting [43]. Figure 5 suggests the wide-angle diffractograms of neat PC and five PC/CN/CaCl 2 -PA-66 after annealing at 190°C for 24 h. The diffraction peaks are appeared at approximate 2θ = 17.2°of these except PC, and simultaneously the fainter diffraction peaks are appeared approximate 2θ = 25.2°. These two sets of peaks are attributed to (201), (020) and (223), (303), (222), respectively, which indicate that the PC belongs to the orthorhombic system [46]. Besides, the intensity of the peaks exhibits a trend of rising and then decreasing with the increase of DI. The interplanar spacing (d hkl ), i.e., the distance between two adjacent parallel crystal planes, can be calculated by equation (3): where λ is the wavelength of the x-ray (1.5406 Å), d hkl is the interplanar crystal spacing, and θ is the Bragg's angle. From table 4, the d hkl of PC/CN/CaCl 2 -PA-66 is almost equivalent, and nearly do not change with the DI, which means that the crystal structure of PC is basically not affected by the DI of the nucleating agent.

Nucleation mechanism of ionized PA-66 by ion-dipole interactions for PC crystallization
In the PC/CN/PA-66 system, PA-66 acts as a heterogeneous nucleating agent to promote PC nucleation. In the PC/PA-66 interfacial layer, there will be partial weak hydrogen bonding between the H in the PA-66 amide  group and the O in the PC carbonate group. From figure 6(a), each hydrogen bond is composed of covalent and ionic components, i.e., H-O bonding and dipole-dipole interactions between the amide group and the carbonate group. The hydrogen bonding improves the compatibility between PC and PA-66 to some extent, which allowing dispersion of PA-66 crystals and induction of PC crystallization [47].
When PA-66 is ionized by CaCl 2 through the solution ionization (section 2.2), as shown in figure 6(b), the interaction between the nucleating agent and the matrix is changed, with some of the hydrogen bonding in the original interface being replaced by stronger ion-dipole interactions, each of which also contains a covalent component and an ionic component. The former is the H-O and Ca-O bonds, while the latter is an ion-dipole interaction between the C=O of PC and the amide group affected by the (1/2)Ca 2+ -Cl − ion pair in CaCl 2 -PA-66, both of which are stronger than the covalent and ionic components of the hydrogen bonding interaction in the PC/CN/PA-66 system, resulting in stronger interactions between the nucleating agent and the substrate and better compatibility [45]. The results manifest in better dispersion of CaCl 2 -PA-66 as a nucleating agent in PC, smaller size and higher number density of the nucleating agent dispersed phase, and the former is more favorable to nucleation, while the latter effectively enhances the nucleation efficiency. Consequently, the PC/CN/CaCl 2 -PA-66 system can lead to the higher crystallinity and smaller grain size of PC compared with the PC/CN/PA-66 system.
The increase of ionization of the nucleating agent didn't enhance the crystallinity of PC monotonically, and the experimental results reveal a trend of increasing and then decreasing. Thus, it is assumed that there are two effects, one positive and two negativities, in the system with the rise of DI, which have different trends and thus have different effects on PC crystallization. The positive effect is the thermodynamic factor, the ion content improves the interfacial compatibility of the two phases, the dispersed phase size of the nucleating agent is smaller, and the number density is higher. This two as a whole, the finer dispersion is accompanied by an increase in the number density, and the former is conducive to heterogeneous nucleation, so that the nucleation effect is better. While the latter increases the nucleation sites, thus nucleation efficiency becomes higher. The negative effect includes both kinetic and thermodynamic factors. From the kinetic aspect, the ionization of the nucleating agent will produce ionic crosslinking, and the ionic crosslinking density will promote accordingly  with the raise of DI, which elevates the viscosity of dispersed phase and is not conducive to its detailed dispersion in the PC matrix. Eventually, the dispersed phase becomes larger in the matrix, the number density decreases, and the efficiency of heterogeneous nucleation decreases. From the thermodynamic aspect, with the increase of the DI, the nucleating agent is more finely dispersed and the number density of particles is higher. As the ionization of the nucleating agent increases, the nucleating agent is more finely dispersed and the number density of particles is higher, but the proportion of amorphous interfacial layer between the nucleating agent and matrix is increasing, and the proportion of crystallizable matrix is decreasing, thus the crystallization ability of PC is reduced. When the nucleating agent ion content is low, the positive effect of ionization is higher than the two negative effects, and the PC crystallinity increases with the increase of ionization. Relatively, when the ionization degree increases to a certain degree, the enhancement of ion crosslinking density makes the nucleating agent viscosity increase significantly, the negative effect of kinetic factors starts to be higher than the positive thermodynamic effect. Considering the particle size of the dispersed phase of the nucleating agent is expanded, and the number density is decreased, the negative effect of kinetics is more significant and dominant, even though the change makes the negative thermodynamic effect diminish. As a result, the PC crystallinity will be decreased with the continued increase of ion content. To prove the above arguments, the modified PC were made into thin films, the CaCl 2 -PA-66 nucleating agents on the surface of the films were dissolved by etching, and then the dispersion of the nucleating agents was observed by SEM to evaluate the pore size and the degrees of dispersion. Figures 7 and 8 demonstrate the SEM photos and the nucleator particle size distribution histograms, respectively.
From figures 8(a)-(c), the particle size distribution is narrowed and the number of small size particles is increased significantly, which causing the average particle size is reduced from 0.33 to 0.20 μm. From figures 8(d), (e), as the DI of the nucleating agents rises, the particle size distribution becomes wider, and the distribution in the larger size interval is expanded gradually. Thus, the average particle size is expanded to 0.34 and further 0.62 μm. The latter far exceeds the particle size distribution of a PA-66 nucleating agent ( figure 8(a)) and has the lowest nucleation efficiency, so its crystallinity is the lowest.
The negative thermodynamic effect, i.e., the increase in the volume of the amorphous interfacial layer due to the raise in the number density of a CaCl 2 -PA-66 nucleating agent, does not seem to play a crucial role in the system. The negative effect of the amorphous interfacial layer at lower DI does not remarkably inhibit the crystallization of PC, while the negative kinetic effect at higher ionization levels reduces the negative thermodynamic effect significantly. In summary, when the DI of the nucleating agent is low, the thermodynamic positive effect is predomination. With the particle size of the nucleating agent gradually narrows and the number density gradually increases, the heterogeneous nucleation effect is also getting better. As the DI of the nucleating agent is enhanced, the kinetic negative effect is gradually more prime. Consequently, the nucleating agent are not easy to disperse uniformly due to the higher ion crosslinking density, the nucleating agent particle size becomes larger, the number density decreases, and the nucleation induction effect monotonically decreases.

Conclusions
To improve the crystallinity of poly(bisphenol-A carbonate) (PC), CaCl 2 -ionized polyamide-66 (CaCl 2 -PA-66) nucleating agents with different degrees of ionization (DIs) were prepared to modify PC under the plasticization of cholesterol nonanoate (CN). The melt crystallization behavior of PC was investigated after annealing at 190°C for 24 h. In the plasticizer one-component modification system, CN induces partial crystallization of PC, but the crystallinity is only 13.8%. In the nucleating agent and plasticizer co-modified PC system, the crystallinity of PC/ CN/non-ionized PA-66 is 19.2%. After the nucleating agent ionized, the crystallinity of PC is further enhanced, which of PC/CN/6.5 mol% CaCl 2 -PA-66 is 28.2%, and moreover PC/CN/11.4 mol% CaCl 2 -PA-66 is 34.5%. However, as the DI continued to rise, the crystallinity of PC represents a decreasing trend, which of PC/CN/19.7 mol% CaCl 2 -PA-66 and PC/CN/37.2 mol% CaCl 2 -PA-66 are, respectively, 25.9% and 16.9%.
The mechanism of the effect of the ionized nucleating agent on PC is proposed. The ionized nucleating agent introduces the ion-dipole interactions in the system, which significantly improve the dispersion of the nucleating agent in the matrix, and the heterogeneous nucleation efficiency of the nucleating agent is effectually enhanced. Nevertheless, the increase in the number density of the nucleating agent leads to an expansion of the volume of the amorphous interfacial layer in the system, which is not conducive to PC crystallization. Moreover, the too high DI (11.4 mol%) of the nucleating agent will increases its ionic cross-linking density and makes it difficult to disperse in the matrix, resulting in a raise in particle size of the dispersed phase of nucleating agent and a decrease in number density, which reduces the nucleation efficiency of PC. Summarily, the nucleating agent with the appropriate DI leads to the best PC crystallization performance. This work proves a new idea to enhance the crystallinity of PC.