Elsevier

European Polymer Journal

Volume 103, June 2018, Pages 293-303
European Polymer Journal

Chemical and physical effects of processing environment on simultaneous single crystallization of biodegradable poly(ε-caprolactone) and poly(l-lactide) brushes and poly(ethylene glycol) substrate

https://doi.org/10.1016/j.eurpolymj.2018.04.009Get rights and content

Highlights

  • PCL and PLLA crystallizable brushes were fabricated by single crystal growth.

  • Miscibility of brushes caused low, high, and no crystallization induction.

  • The nature of neighboring brushes affected crystallization induction.

Abstract

Through growth of poly(ethylene glycol) (PEG)-b-poly(ε-caprolactone) (PCL) and PEG-b-poly(l-lactide) (PLLA) single crystals from dilute solution, the crystallization of crystallizable PCL and PLLA brushes in homo and mixed-brush nanostructures was investigated. Using various graftings, the physical and chemical environmental effects were studied on the brush crystallization. The brush interactions were investigated as chemical environmental effects. The physical environmental effect was also determined by the amorphism/crystallinity and rigidity/flexibility of neighboring brushes. For this purpose, different single crystals were designed with mixed-brushes of amorphous-crystalline (polystyrene (PS), poly(methyl methacrylate) (PMMA), PCL and PLLA), double crystalline (PCL/PLLA), and rod-crystalline polyaniline (PANI)/PCL or PLLA. The type of surrounding brushes and their miscibility with crystallizable brushes, inherent amorphism or crystallinity, and flexibility or rigidity were effective on the behavior of crystallizable brushes and induced crystallization procedure. The neighboring brushes could also prevent the PLLA grafts from crystallization in the presence of PMMA.

Introduction

The block copolymer single crystals have attracted attention in polymer brushes [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], brush regimes [1], [2], [3], [11], surface patterning [1], [12], [13], [14], [15], [16], [17], [18], crystallization in nano-confined environment [19], [20], [21], [22], [23], [24], [25], [26], biodegradation mechanism [27], [28], [29], effect of physical and chemical environments on single crystal growth [1], [2], [16], [17], [18], etc. The crystallization mechanism and amorphous blocks effect were also investigated [5], [30], [31]. The study of single crystal structures in double crystalline block copolymers is difficult and complicated, thereby some few research teams have focused on it [27], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52]. The double crystalline block copolymers are divided into two total categories, i.e., the block copolymers with similar melting points such as PEG-b-PCL [27], [32], [33], [34], [42], [46], [47], [48], [50] and poly(4-methyl-1-pentene)-block-poly(l-lactide) (sPMP-PLLA) [35] and the block copolymers having significantly different melting points including polyethylene (PE)-b-PCL [39], PE-b-PEG [44], PLLA-b-PCL [36], [43], [46], [47], [50], PLLA-b-PEG [37], [38], [45], [46], [47], [49], [50], PE-b-poly(3-butylthiophene (P3BT) [41] and PEG-b-poly(3-hexylthiophene (P3HT) [51], [52]. For PEG-b-PCL crystals grown from dilute solution, it was reported that the block with higher molecular weight first crystallized, and the other block was either amorphous or crystalline in especial conditions [27], [32], [33], [34], [42], [46], [47], [48], [50]. Van Horn et al. [32], [42] using the selective solvents and homopolymer seeds changed the crystallization priority, which was dictated by the weight fraction of different blocks. Moreover, while crystallization of second block on the single crystalline substrate of first block, the height and grafting density of brushes affected their crystallization [42]. The single crystals of PEG-b-PLLA block copolymers have been only developed in the melt state [37], [38], [45], [46], [47], [49], [50].

Biodegradable aliphatic polyesters such as PCL and PLLA have received considerable attention over the past decades in the medical and biomedical applications [47], [50]. The rate of biodegradation and drug release kinetics are strongly influenced by crystallinity [47], [50], [53], thereby it is of critical importance that such polymers should be well-characterized from view point of crystallinity in their grafted brushes. The crystallization of these polymers decreases the biocompatibility and biodegradability, thereby the recognition of physical and chemical conditions which manipulate the crystallinity and amorphism of these polymers is vital. In the current work, through growth of PEG-b-PCL and PEG-b-PLLA single crystals from dilute solution, the crystallization of crystallizable PCL and PLLA brushes in homo- and mixed-brushes was investigated. In addition to grafting density of polymer brushes [42], the effect of neighboring brushes was introduced. Indeed, by using various neighboring brushes, the physical and chemical environmental impacts of crystallizable brushes on the crystallization process were studied. The chemical environmental influence was investigated by considering the interactions of crystallizable brushes with the surrounding brushes. The physical environmental effect was also determined by the amorphism/crystallinity and rigidity/flexibility of neighboring brushes. To this end, the homo-brush single crystals sandwiched between the crystallizable coily PCL and PLLA brushes as well as the mixed brushes of amorphous-crystallizable PS or PMMA/PCL or PLLA, crystallizable-crystallizable PCL/PLLA, and rigid rod-crystallizable PANI/PCL or PLLA were designed.

Section snippets

Experimental

The diblock copolymers of PEG5000-b-PS [6], [12], [30] and PEG5000-b-PMMA [12], [30] were synthesized by atom transfer radical polymerization (ATRP) with the polydispersity indices (PDI) of 1.12–1.17 and 1.19–1.21, respectively. The PEG5000-b-PANI diblock copolymers were synthesized by an interfacial polymerization with a strong dopant of potassium hydrogen biiodate (PHD). The PANI blocks synthesized by this method were fibers with a nanometer diameter range. Details of synthesis method

Crystallizable homo-brushes

The crystallizations of PCL and PLLA homo-brushes in PEG-b-PCL4700, PEG-b-PCL9100, PEG-b-PLLA7800 and PEG-b-PLLA13300 single crystals grown from dilute solutions were thoroughly investigated. Solubility parameter of amyl acetate, PEG, PCL and PLLA are 8.5, 9.9, 9.4 and 10.9 (cal/cm3)1/2, respectively. Hence, amyl acetate was a poor, theta, and very poor solvent for PEG, PCL and PLLA, respectively [40], [42], [58], [59]. Fig. 1 shows the schematic crystallization steps of crystallizable brushes.

Conclusions

By using various neighboring brushes, the environmental effects were studied on the crystallization procedure. The chemical and physical environmental impacts were investigated from the perspectives of crystallizable brushes interactions with the surrounding brushes, amorphism/crystallinity, and rigidity/flexibility of neighboring brushes. By comparing the crystallization temperatures of crystallizable brushes in their homo- and mixed-brushe single crystals, it was deduced that the type of

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