Cellulose Acetate Thermoplastics with High Modulus, Dimensional Stability and Anti-migration Properties by Using CA-g-PLA as Macromolecular Plasticizer

Cellulose diacetate (CDA) can be melt-processed to produce numerous and widely-used plastic products. However, due to the high glass transition temperature (Tg) of CDA, the addition of up to 30 wt% of micromolecular plasticizers is indispensable, which significantly reduces the dimensional stability and raises safety concerns from the migration of plasticizers. In this work, a series of CDA-graft-poly(lactic acid) (CDA-g-PLA) copolymers were synthesized by ring-opening polymerization of lactide onto the hydroxyl groups of CDA. The resultant CDA-g-PLA copolymers possess adjustable degrees of substitution (DSPLA) and side chain length (DPPLA) by controlling the reaction time and feed ratio. The Tgs and thermal flow temperatures (Tfs) of CDA-g-PLA strongly depend on DPPLA, such as the Tgs decrease linearly with the increase of DPPLA. The CDA-g-PLA copolymers with the DPPLA of 3–9 can be directly processed to transparent plastics by melt processing without any external plasticizers, because of their low Tfs of 170–215 °C. More impressively, the CDA-g-PLA can act as the macromolecular plasticizer. The obtained CDA/CDA-g-PLA has higher storage modulus, flexural modulus and Young’s modulus than the commercial CDA plasticized with triethyl citrate. In addition, the CDA/CDA-g-PLA exhibits high dimensional stability and anti-migration property. During a long-term treatment at 80 °C and 60% humidity, the CDA/CDA-g-PLA can retain the initial shape. Therefore, this work not only proposes a facile method for achieving a direct thermoplastic processing of CDA, but also provides a macromolecular plasticizer for CDA to make lightweight, stable and safer biobased thermoplastics.


INTRODUCTION
Eco-friendly, renewable, and recyclable materials have received an increasing attention, due to the escalating environmental pollution. [1−6] Cellulose diacetate (CDA) obtained by the acetylation of the biopolymer cellulose is one kind of the commercial and important polymer materials. CDA with an average degree of substitution (DS) of acetyl groups of 2.2−2.7 per glucose unit is industrially produced by a heterogenous acetylation of cellulose followed by a hydrolysis process. Since CDA exhibits excellent solubility and formability, via a solution processing strategy, it can be readily processed into fibers, films and coatings, which have been used as fabrics, cigarette filters, separation films, printing ink, paint, etc. [7−12] More interestingly, CDA possesses thermoplastic character with the addition of external plasticizers. Via a thermoplastic processing, CDA can be shaped into various sheets, plates, strips and other plastic forms to fabricate eyeglass frames, toys, tool handles, and so on. [13,14] In addition, CDA-based materials have high rigidity, moderate impact strength, excellent transparency, special absorption ability and good selective permeability. Hence, they have been extensively used in many fields. [15−17] In CDA, there are abundant polar groups, including hydroxyl and acetyl groups, which form strong intermolecular interactions. As a result, the polymer chain of CDA is difficult to move, resulting in a high glass transition temperature (T g ) of 190 °C. In addition, CDA does not display crystallization behavior and usually exists in an amorphous form. Therefore, in the absence of external plasticizers, CDA cannot be melt-processed and degrades at a high temperature. [18] It is of significant importance to plasticize CDA and reduce the T g in order to achieve a thermoplastic processing of CDA.
Two strategies, including physical blending (external plasticizing) and chemical modification (internal plasticizing), have been developed to plasticize CDA. The external micromolecular plasticizer is industrially used in the thermoplastic processing of CDA. Generally, 30 wt% of micromolecular plasticizer is added during the thermoplastic processing of CDA. The common plasticizers are phthalates (dimethyl phthalate, diethyl phthalate, dioctyl terephthalate, etc.), epoxides (epoxidized soybean oil, etc.), glycerol esters (glycerol diacetate, glycerol triacetate, etc.) and citrates (triethyl citrate, acetyl triethyl citrate, etc.). [19−22] Recently, ionic liquids, such as 1-butyl-3-methylimidazolium chloride and 1-butyl-3-methylimidazolium tetrafluoroborate, were used as new and efficient plasticizers for CDA in lab-scale studies. [23,24] However, since most of the micromolecular plasticizers tend to migrate out and to be released into the environment during the preparation and usage, the corresponding products have gradually deteriorative properties and short service life. Moreover, the migrating chemicals even cause serious hazards to the human health and environment. [25,26] To overcome these disadvantages of the micromolecular plasticizers, some polymers with low T g s have been employed to plasticize CDA, such as poly(ethylene glycol) (PEG), modified starch, polycaprolactone (PCL) and poly(lactic acid) (PLA). [20] The plasticizing effect of PEG200 is comparable to that of citrate plasticizers, resulting in the excellent fracture toughness in the final product. Unfortunately, due to the low molecular weight, PEG200 is prone to migrate. In addition, the high molecular-weight PEG1500, modified starch, PCL and PLA have poor compatibility with CDA.
Chemically introducing the substituents containing longchain groups can effectively reduce T g s of cellulose derivatives to achieve a thermoplastic processing. [27−32] Tedeschi et al. [33] modified CDA with oleate groups in a mixture of trifluoroacetic acid and trifluoroacetic acid anhydride. The obtained cellulose acetate oleate mixed esters have low T g s, high oxygen barrier properties and mechanical behaviors closer to ductile materials. Iji et al. [34,35] reported the bonding of CDA with cardanol group containing a terminal long alkyl chain. The synthesized cardanol-bonded CDA has low T g , which is as low as 140 °C by adjusting the degrees of substitution (DS) of cardanol. Boulven et al. [36] found that the substituent with the most effective plasticization effect consisted of a linear aliphatic segment as the spacer and a bulky and rigid moiety (particularly aromatic moiety) as the terminal. In addition, to graft flexible side chains as internal plasticizes is also an effective method for achieving a thermoplastic processing of CDA. Poly(3-hydroxy butyrate) (PHB), poly(valerolactone) (PVL), PCL, PLA, and PEG have been introduced into CDA. [37,38] However, in order to achieve sufficient plasticization effect, it generally needs a high content of grafted side chains, which will cause a considerable change in the properties of CDA-based materials.
In this work, we synthesized a series of CDA-graft-poly(lactic acid) (CDA-g-PLA) copolymers with controllable structures.
Subsequently, the effect of CDA-g-PLA structures on their thermal properties and compatibility was systematically researched. The resultant CDA-g-PLA copolymers were directly processed to transparent plastics by thermoplastic processing without any external plasticizers. Moreover, they acted as a macromolecular plasticizer for CDA to improve mechanical strength, stability and anti-migration property.

Materials
CDA was supplied by Sichuan Pushi Acetati Co., Ltd. The DS of acetyl groups in CDA was 2.44 as determined by 1 H-NMR. Llactide (LA) was purchased from J&K Scientific Ltd. 4-Dimethylaminopyridine (DMAP) with a purity of 99.5% was provided by Haili Chemical Industry Co., Ltd. All other chemicals used were of analytical grade, and were purchased from Sinopharm Chemical Reagent Co., Ltd.

Synthesis of CDA-g-PLA
A typical polymerization procedure was used as illustrated in Fig. 1(a). Firstly, dried CDA powder (4.0 g) was dissolved in N,N-dimethylformamide (DMF) to obtain a 5 wt% CDA/DMF solution. Subsequently, 0.915 g of DMAP and a certain amount of L-lactide were added. The reaction was carried out under nitrogen atmosphere at 80 °C for different time. The resultant homogeneous solution was added dropwise into the methanol/water mixture (V/V, 1/1) to terminate the reaction. The precipitate was filtered, washed thrice with methanol, and dried. Then, the precipitate was dissolved in DMF and precipitated with water/toluene mixture (V/V, 1/1) to remove the homopolymer of L-lactide. Finally, the product was filtered and dried under vacuum at 60 °C for 24 h.

Preparation of CDA/CDA-g-PLA Blend Films
CDA (1.0 g) and CDA-g-PLA (1.0 g) were simultaneously dissolved in DMF to get a CDA/CDA-g-PLA/DMF solution. Then, the CDA/CDA-g-PLA blend films were obtained by the solvent evaporation and drying at 80 °C for 12 h.

Melt Processing of CA-g-PLA Copolymers and CDA/CDA-g-PLA Blends
The dumbbell samples were prepared by injection molding with a Haake Mini jet apparatus. The injection temperature was in a range of 200−220 °C, the mold temperature was controlled at 60 °C, and the pressure was 80 MPa. The disk samples and films were prepared by hot pressing at 180−200 °C. CDA/CDA-g-PLA fibers were fabricated by a melt spinning process using a Haake-SWO 556-0031 melt flow speed indicator. About 5.0 g of CDA/CDA-g-PLA was put into the barrel of the equipment, heated at a rate of 40 °C/min up to 180−220 °C, kept at this temperature for 2 min, and then extruded through a capillary under a constant load of 1.2 kg. The extruded fiber was taken up at a speed of 10 m/min at room temperature.

Characterization
1 H-NMR spectra were acquired on a Bruker AV-400 NMR spectrometer with 16 scans at room temperature in DMSO-d 6 . A few drops of trifluoroacetic acid-d 1 were added to shift the signals of the active hydrogens downfield.
Differential scanning calorimetry (DSC) was conducted on a Q2000 differential scanning calorimeter (TA Instruments, USA) under a nitrogen atmosphere. To provide the same thermal history before the measurements, each sample was heated to 240 °C at a scanning rate of 20 °C/min and maintained at 240 °C for 5 min. Then, the samples were cooled to 20 °C, kept for 5 min. Then, the samples were heated from 20 °C to 240 °C at a rate of 20 °C/min again. All the reported T g s were calculated from a second heating cycle.
Thermal flowing behaviors were observed by an Olympus BX5.1 optical microscope equipped with a Linkam THMS 600 hot-stage device. A small piece of the sample was sandwiched between two cover glasses and heated from 20 °C to 260 °C at a rate of 10 °C/min. The whole process was monitored by recording the photos at the desired temperature intervals. The temperature, at which the cellulose ester was completely transparent and flowed, is regarded as the thermal flowing temperature (T f ).
Thermogravimetric analysis (TGA) was carried on Perkin-Emler Pyris 1 thermogravimetric analyzer in nitrogen atmosphere. Each sample was heated from 50 °C to 750 °C at a rate of 20 °C/min. Dynamic mechanical analysis (DMA) was carried out on a TA Instruments DMA Q800. The specimen was a rectangular strip with dimensions of 17 mm × 12 mm × 1 mm, which was obtained by injection molding process. The measurements were performed in the single cantilever mode at a constant frequency of 1 Hz over a temperature range from −50 °C to 250 °C and a heating rate of 3 °C/min in nitrogen atmosphere.
The cross-sectional morphologies of CDA/CDA-g-PLA blends were observed with a scanning electron microscope (SEM) (JSM-6700F, JEOL, Japan) at an accelerating voltage of 10 kV. The samples were coated with platinum before observation.
Tensile test and bending test were performed on a universal testing machine (Instron 3365, Instron, USA) with a tensile speed of 5 mm/min or a bending speed of 2 mm/min.

Synthesis and Characterization of CDA-g-PLA
The hydroxyl groups can effectively trigger the ring-opening polymerization of L-lactide. [39−41] Thus, a series of CDA-g-PLA copolymers were synthesized by using the hydroxyl groups in CDA as the initiator and the DMAP as the catalyst. CDA-g-PLA copolymers with adjustable structures were obtained via controlling the reaction conditions, such as the reaction time and the molar ratio of L-lactide (LA) to anhydroglucose unit (AGU) (Fig. 1a). 1 H-NMR spectra (Fig. 1b) and FTIR spectra (Fig. 1c) confirm that CDA-g-PLA copolymers with different structures are fabricated. As shown in Fig. 1(b), the signal at 5.0−5.  Table 1. Compared with FTIR spectrum of PLA, FTIR spectra of CDA-g-PLA copolymers show the characteristic C-O-C band at 1030 cm −1 (Fig. 1c), indicating that the products are not a homopolymer of LA and the grafting reaction occurs on the hydroxyl groups of CDA. Compared with FTIR spectrum of CDA, the bending vibration band of CH 3 at 1430 cm −1 in FTIR spectra of CDA-g-PLA gives a blue shift as the increase of MS PLA , meanwhile the rocking vibration bands of CH 3 at 868 and 756 cm −1 appear, confirming the formation of PLA side chain. [39−42] Moreover, in FTIR spectra of CDA-g-PLA copolymers, there is a stretching vibration band of C＝O at 1740 cm −1 , a stretching vibration band of CH 3 at 1370 cm −1 , and an asymmetric stretching vibration band of C-O-C at 1210 cm −1 , which are the characteristic bands of CDA. These results prove that PLA chains are successfully grafted on the CDA chains. Reaction conditions, including LA/AGU molar ratio and reaction time, significantly affect the chemical structure of CDAg-PLA copolymers (Table 1 and Fig. S1 in the electronic supplementary information, ESI). For example, under the reaction time of 6 h, DP PLA and MS PLA exhibit an approximately linear growth as the increase of LA/AGU molar ratio. The DS PLA increases rapidly at first, then the increase rate of DS PLA becomes slow when the LA/AGU molar ratio is higher than 8/1. Under the LA/AGU molar ratio of 20/1, the DP PLA increases linearly with the prolongation of reaction time. The DS PLA and MS PLA increase rapidly at first, then remain unchanged or increase slowly when the reaction time exceeds 10 h. Therefore, CDA-g-PLA copolymers with different DS PLA , DP PLA , and MS PLA have been synthesized by controlling the LA/AGU molar ratio and reaction time.

Thermal Properties of CDA-g-PLA
The CDA-g-PLA copolymers with different DP PLA exhibit the diverse initial degradation temperatures (T onset s) ( Table 1 and Fig. S2 in ESI). The TGA curves of CDA-g-PLA copolymers with the short PLA side chains (DP PLA < 5) are similar to that of CDA.
The T onset s of these CDA-g-PLA copolymers are above 350 °C. The CDA-g-PLA copolymers with the long PLA side chains (DP PLA > 5) have slightly low T onset s, which are higher than 330 °C. Therefore, all CDA-g-PLA copolymers have high thermal stability.
The T g s of CDA-g-PLA copolymers strongly depend on DP PLA , while have no relation to DS PLA (Table 1 and Fig. 2a). When the CDA-g-PLA copolymers have similar DS PLA , their T g s linearly decrease as DP PLA increases, as shown in Fig. 2(b). The lowest T g of CDA-g-PLA with a DP PLA of 8.87 is 78 °C. When the CDA-g-PLA copolymers have similar DP PLA , the T g s remain unchanged as DS PLA changes. For instance, CDA-g-PLA-12 and CDA-g-PLA-13 with similar DP PLA have different DS PLA , 0.53 and 0.29, respectively. They display almost same T g values, 164 and 163 °C, respectively. Thus, the long PLA side chains have a better internal plasticization effect, and can effectively reduce the T g s of CDA-g-PLA copolymers.
Neat CDA has a high T g of 193 °C and a high T f of 260 °C. Moreover, the thermal flowing property of CDA at 260 °C is poor. So, in order to achieve a thermoplastic processing of CDA, 20 wt%−30 wt% of micromolecular plasticizers have to be added to reduce the T f and improve the thermoflowability. Due to the internal plasticization effect of PLA side chain, most of the CDA-g-PLA copolymers exhibit thermal flowing behaviors at the temperature below 230 °C (Table 1 and Fig. 2c). The T f s decrease with the increase of MS PLA and W PLA .
The lowest T f is 170 °C. Therefore, most of the CDA-g-PLA copolymers can be directly shaped into uniform and transparent plastic products via thermal processing process, such as injection molding, hot pressing, as shown in Fig. 2(d).

Compatibility of CDA-g-PLA with CDA
The compatibility between CDA-g-PLA copolymers and CDA extremely relies on the W PLA of CDA-g-PLA. When the W PLA of CDA-g-PLA is below 30 wt%, such as CDA-g-PLA-1, CDA-g-PLA-2, CDA-g-PLA-5 and CDA-g-PLA-6, CDA/CDA-g-PLA blend films show good uniformity and excellent optical transparency (Figs. 3a and 3c). In addition, the cross sections of CDA/CDA-g-PLA blend films are uniform and consistent (Fig. 3b) with no phase separation. Moreover, there is only one T g in DSC curves (Fig. 3d). Hence, CDA-g-PLA copolymers with the W PLA below 30 wt% have excellent compatibility with CDA. When the CDAg-PLA-9 with a W PLA of 39.4 wt% is blended with CDA, the SEM image reveals that there is a microphase separation. Although the CDA/CDA-g-PLA-9 film is still uniform and transparent, there are two T g s in the DSC curve. When the W PLA of CDA-g-PLA is up to 54.2 wt%, such as CDA-g-PLA-10, the obtained CDA/CDA-g-PLA-10 film is opaque. Besides, an obvious phase separation phenomenon occurs, and there are two T g s in the DSC curve. Therefore, the compatibility between CDA-g-PLA and CDA decreases as the increase of W PLA .

Thermoplastic Processing and Properties of CDA/CDA-g-PLA Blends
Since CDA-g-PLA copolymers have low T g s, low T f s, and tunable compatibility with CDA, we used CDA-g-PLA copolymers as the macromolecular plasticizer to achieve a thermoplastic processing of CDA ( Fig. 4 and Fig. S3). When CDA-g-PLA-6 with a good compatibility with CDA is used, the resultant CDA/ CDA-g-PLA-6 blend can be hot pressed into transparent film at 180 °C, be injected into the disk and rectangular samples at 220 °C, and be spun into fiber by melt extrusion at 220 °C. The CDA-g-PLA-9 and CDA-g-PLA-10 with a poor compatibility with CDA can also act as the macromolecular plasticizer. As shown in Fig. 4 and Fig. S3 (in ESI), CDA/CDA-g-PLA-9 and CDA/CDA-g-PLA-10 can be processed into opaque plastics by hot pressing and injection molding. Therefore, the CDA-g-PLA can act as the macromolecular plasticizer to successfully realize the thermoplastic processing of CDA. Because the high molecular-weight CDA-g-PLA copolymers are used as the plasticizer, the obtained CDA/CDA-g-PLA exhibits extremely higher storage modulus than the CDA plasticized by 30 wt% triethyl citrate at room temperature, as shown in Fig. 5(a) and Table S1 (in ESI). More impressively, at a high temperature of 130 °C, the CDA/CDA-g-PLA-6 even has a high storage modulus, which is comparable to that of CDA/triethyl citrate at 25 °C. In addition, tensile and flexural tests show that CDA/CDA-g-PLA-6 has the higher flexural modulus and Young's modulus than CDA/triethyl citrate. Thus, the CDA-g-PLA copolymers as the plasticizer can remarkably improve the mechanical property and thermal stability of CDA-based thermoplastics, indicating that the CDA/CDA-g-PLA has a huge potential in lightweight and high-strength thermoplastics.  The common CDA/triethyl citrate plastics are easy to deform under high humidity and/or high temperature environment due to the water absorption and hence, showing poor dimensional stability in the real environment. Moreover, the micromolecular plasticizer in CDA/triethyl citrate plastics tend to leach out once the CDA/triethyl citrate plastics come into contact with water or other solvents. As shown in Fig. 5(c), CDA/triethyl citrate film gives a visible bend when it is exposed to a condition of 80 °C and 60% humidity for 1 h. When the CDA/triethyl citrate films are immersed in water and ethanol, their weight rapidly decreases, indicating that the micromolecular plasticizer leaches out (Fig. 5d). The high molecular-weight CDA-g-PLA copolymers as the plasticizer can effectively overcome these problems. As shown in Fig. 5(c), CDA/CDA-g-PLA-6 film can keep its initial shape without any deformation phenomenon after being placed at 80 °C and 60% humidity for 4 h. Moreover, the CDA/CDA-g-PLA-6 film is very stable in water and ethanol without any migration phenomenon (Fig. 5d). Thus, when the high molecular-weight CDA-g-PLA copolymers act as the plasticizer, the CDA/CDA-g-PLA exhibits high dimensional stability and anti-migration property, which provides an opportunity to construct lightweight, stable and safer biobased thermoplastics.

CONCLUSIONS
In this work, we synthesized a series of CDA-g-PLA copolymers by adjusting the reaction time and the molar ratio of LA/AGU. The resultant copolymers have significant difference in structure, thermal behavior, and compatibility with CDA. To be specific, the T g s of CDA-g-PLA copolymers decrease linearly with the increase of DP PLA , and the T f s strongly depend on the PLA content. The CDA-g-PLA copolymers with the DP PLA of 3−9 can be directly processed to transparent plastics by thermoplastic processing without any external plasticizers, based on their low T f s of 170−215 °C. Additionally, the prepared CDA-g-PLA was tested as macromolecular plasticizer for CDA. The compatibility between CDA-g-PLA and CDA decreases as the increase of PLA content. Then, the optimal CDA-g-PLA plasticizer was chosen to realize the thermoplastic processing of CDA. The obtained CDA/CDA-g-PLA can be readily processed into different shapes by traditional thermoplastic processing. The CDA/CDA-g-PLA plastics overcome the disadvantages of the CDA plastics containing a micromolecular plasticizer. They exhibit high modulus, excellent dimensional stability and anti-migration property. Therefore, the CDA thermoplastics plasticized with CDA-g-PLA can be used as safe, stable and lightweight biopolymer-based thermoplastics.

Electronic Supplementary Information
Electronic supplementary information (ESI), which includes effects of the reaction conditions on the structure of CDA-g-PLA copolymers, TGA curves, photomicrographs, and mechanical properties, is available free of charge in the online version of this article at https://doi.org/10.1007/s10118-020-2470-2.