PLLA-PDLA

A novel material made of biodegradable polymer reinforced with modified calcium phosphates (TCP) particles will be proposed to be used in fabrication of novel constructs for the repair of critical-sized bone defects. Several composite materials made of PLLA/PDLA or PCL reinforced with TCP micro and nanoparticles will be discussed. 

Stereocomplex PLLA/PDLA LLLLLLLL mixed with DDDDDDDD... 

Poly(lactic acid)s can be synthesized by two methods (i) polyeondensation of lactic acids and (ii) ROP of LAs [1, 3, 8, 54, 55], Figure 8.3 shows the synthesis of PLLA, PDLA, and their block copolymers [11], Tin-based catalysts such as tin (II or IV) chloride and tin (II) bis-2-ethyUiexanoic acid (stannous octoate or tin octoate) were reported to effectively increase the molecular weight of the resulting PLAs up to an order of 10 and 10 g mol for polyeondensation [56,57] and ROP [58-61], respectively. Moreover, bulk polymerization in the presence of these catalysts aids in avoiding racemization and transesterification during the ROP of LA. 

Figure 8.3 Synthesis of PLLA, PDLA, and their biock copoiymers 111], (Reproduced with permission from ref. 111], Copyright 2005 Wiiey-VCH Veriag GmbH Co. KGaA, Weinheim.)...

PDLA or PLLA when solely stereocomplexation takes place [107], However, spherulites with complicated morphologies are evident when stereocomplexation and homo-crystallization occur simultaneously [104,107,108]. Maillard and Prud homme observed mirror-image stereocomplex crystalline assemblies for PLLA- and PDLA-rich PLLA/PDLA mixtures [109]. As observed for crystalline polymers, PLLA spherulites become smaller with decreasing temperature and time [110,111]. 

As stated above, the hydrolytic degradation resistance of PLA-based materials was enhanced by stereocomplex formation. However, even in the molten state (i.e. above the I m of the stereocomplex crystallites), the PLLA/PDLA blend has a higher thermal resistance than neat PLLA or PDLA [275,276]. 

Table 8.10 Interplane distance (d) values of P(L-2HB)/PDLA hetero-stereocomplex, PLLA/PDLA and P(L-2HB)/P(D-2HB) homo-stereocomplexes 149,501. (Reproduced with permission from ref. [49]. Copyright 2009, American Chemical society and from ref 150]. Copyright 2010, American Chemical Society.)...

The stereocomplex formation of PLLA and PDLA is enhanced in the presence of stereoblock poly(lactic acid) [419], Stereocomplex formation was proposed to be a novel method of controlling the protein- and cell-adhesive properties of biodegradable matrices composed of PEG-PLA copolymers [420], A triangular growth mechanism was proposed for the PLLA/PDLA stereocomplex [95], and the nucleating mechanism is not altered by the PLLA/PDLA blending ratio [421],... 

PLLA, PDLA, PLDLA Rod diam. 2 mm, 1 10mm Mn 2.2 PBS (pH 7.2) containing either porcine liver esterase (PLE), wheat germ lipase (WGL), porcine pancreatic lipase (PPL), or Rhizopus delemar Wpase (RDL) 37 Wt. loss Degradation rate RDL>PPL>PLE>WGL>PBS Copolymers degrade faster than homopolymers 23... 

Table 1.3 Thermal analysis data for PLLA, PDLA, PDLLA, and PLDLLA [32, 50-58].

The PLLA/PDLA stereocomplex, which is another crystal modification of PLA, was first discovered by Ikada and coworkers [80]. Its structure and physical properties have been studied using a number of different techniques, including infrared spectroscopy [82], optical microscopy [83], calorimetry [84], and X-ray diffraction [85]. Recent reviews by Tsuji and Fukushima et al. summarize the main properties of the stereo complex [86, 87]. 

Figure 5.14 (a) WAXS profiles of a stretched PLLA/PDLA blend recorded upon heating. a, a, and denote the a -, a-fbrm homo-crystals, and stereocomplex crystals, respectively. The strong peaks around 20 = 16.5° and 19.0° are the characteristic reflections of the a - and a-form homo-crystals of PLA. The three peaks around 20 = 11.1°, 20.0°, and 23.2°... 

PLLA-b-PEG-b-PLLA/PDLA-b-PEG-b-PDLA SC, thermo-stable [35]... 

Figure 2.2 WAXD spectrum of the PLLA/PDLA blend after crystallization in the second cycle of Figure 2.1 (the sample was taken out at 140 °C for WAXD measurements).

Tsuji et al. investigated the effects of molar mass, l-LA content and average l-LA and d-LA sequence length on the enzymatic degradation of various PLLA/PDLA blends in the presence of proteinase K. The authors observed that enzymatic hydrolysis of PLLA proceeds via both endo and exo chain scission. Non-blended PLA films are enzymatically hydrolyzable when l-LA content and l-LA sequence length are higher than 0.3 and 3, respectively, and d-LA sequence length is lower than 10. 

Lee et al. used the Langmuir film balance technique to determine the hydrolytic kinetics of stereo-complex monolayers formed from PLLA/PDLA mixtures spread at the air-water interface. The hydrolysis of the mixture monolayers under basic conditions is slower than that of individual PLLA or PDLA monolayers, depending on the composition or the degree of com-plexation. In the presence of proteinase K, the hydrolysis rate of mixture monolayers with >50 mol% PLLA is much slower than that of the singlecomponent PLLA monolayer. The monolayers formed from mixtures with <50 mol% l-PLA do not show any degradation. It is concluded that the slower hydrolysis of mixture monolayers is mainly due to the strong interaction between PLLA and PDLA chains, which prevents the penetration of water or enzyme into the bulk. In an in vivo study on the biocompatibility of PLLA and stereo-complexed nanofibres by subcutaneous implantation in rats, Ishii et al. also observed that stereo-complexed nanofibres exhibit slower degradation than PLLA. ... 

PLLA/PDLA Stereo-complex allows enhancing the thermal-resistance and mechanical properties as well as the hydrolytic stability of PLA-based materials. In the past decades, a great deal of work has been performed to exploit potential biomedical applications of PLA stereo-complex as shown in Table 2.1. ... 

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