Can the Glass Transition Temperature of PLA Polymers Be Increased?

Chapter 14

Can the Glass Transition Temperature of PLA Polymers Be Increased? K. Marcincinova Benabdillah, M . Boustta, J . Coudane, and M . Vert Downloaded by UNIV OF PITTSBURGH on May 3, 2015 | http://pubs.acs.org Publication Date: January 15, 2001 | doi: 10.1021/bk-2000-0764.ch014

C.R.B.A. E S A C.N.R.S. 5473, Faculty of Pharmacy, Montpellier I University, 15, av. Charles Flahault, 34060 Montpellier, France

Hydrolyzable lactic acid-based aliphatic polyesters are among the most promising environment-friendly degradable polymers for temporary commodity applications. The properties of the semi– crystalline members of the family are comparable to those of many presently used, industrial thermoplastic polymers. However, a glass transition temperature below 60°C and a rather low viscosity above T are among the weaknesses of PLA polymers and stereocopolymers. In the past years, many copolymers have been, studied. This paper reviews the main linear degradable copolymers of lactic acids reported in literature and discusses the influence of the comonomers on thermal properties, in particular glass transition temperature which is to be considered as one of the key characteristics for the development of PLA-based commodity devices. g

Introduction In the sixties US companies patented sutures based on poly(glycolic acid) (PGA), and glycolic acid-rich poly(lactic acid-co-glycolic acid) polymers (PLAXGAY where X=percentage of L-lactyl units and Y=percentage of glycolyl units, as suggested by Vert (1)). This was the beginning of the search for synthetic polymers which can degrade after use, a characteristic typical of natural biopolymers. Soon after, poly(lactic acid)s (FLAX) were proposed as bioresorbable matrices for biomedical and pharmaceutical uses (2, 3). High molecular weight PLA are synthesized by ring-opening polymerization of lactides according to the following reaction : 200

© 2000 American Chemical Society In Polymers from Renewable Resources; Scholz, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

201

initiator

η CH "

C—Ο

3

Downloaded by UNIV OF PITTSBURGH on May 3, 2015 | http://pubs.acs.org Publication Date: January 15, 2001 | doi: 10.1021/bk-2000-0764.ch014

Ο

CH

3

H

O

CH 0 3

[_o4'4-o-i*-l I CH

I Η

3

Many initiators can be used (4-6) and many mechanisms, namely cationic, anionic, coordination-insertion, etc. have been proposed. In most cases the exact mechanism of polymerization and corresponding chain structures are still questioned, especially for the industrially used initiating species, namely stannous octoate, zinc metal or zinc lactate (7). Because of their excellent biocompatibility and mechanical properties semicrystalline polylactides are of particular interest for orthopaedic and maxillo facial bone surgery (8-11). The osteosynthetic devices exposed to dynamic stresses in animal and human bodies require very pure polymers of high tacticity and crystallinity (12). On the other hand, matrices for drug delivery systems are generally prepared from amorphous PLA stereocopolymers and PLAGA copolymers (13-16). Micro- and nanoparticles, micro- and nanocapsules, films and implants were considered for the delivery of many bioactive compounds, namely anticancer agents, hormones, anesthetics and antibiotics (17-20). For all these therapeutic applications, the polymers work at body temperature i.e. at 37°C, a value reasonably lower than the glass transition temperature, c.a. 55-60°C, of most of the members of PLA family. Therefore, the glass transition is not a source of major problems for this type of applications, unless uptake of water decreases the glass transition below body temperature. Therapeutic applications do not constitute the only field of interest of degradable and bioresorbable polymers. There are many other applications relevant of the concepts of temporary uses and elimination after use. Insofar as the temporary commodity applications are concerned, bacterial poly(hydroxy butyrate), PHB, was for a long time considered as the most promising degradable aliphatic polyester. Nowadays, PLA is supplanting PHB and is considered as the degradable polymer having the highest potential to be industrially developed after the pilot scale facilities set up by Cargill Dow Polymers in USA, Mitsui in Japan, and Neste in Europe. However, environmentally degradable polymers aimed at commodity applications have to fulfill rather severe thermal requirements related to the larger temperature range to be faced outdoor. With this respect, the glass transition temperature is one of the most important characteristics of a thermoplastic material. It must be high enough to allow a device to preserve its initial form as it is the case for poly(methyl methacrylate) and poly(ethylene terephtalate) whose T values are above 100°C. The glass transition temperature of polymers is generally determined by differential scanning calorimetry (DSC). However broad-line NMR and mechanical relaxation measurements are also used. Jamshidi (21) compared the glass transition temperatures of different polylactides determined by the three methods and concluded to "a fairly good agreement of the results". T was found almost g

g

In Polymers from Renewable Resources; Scholz, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

202 independent on the configurational structure although many other factors can affect this values (molecular weights, polydispersity, residual solvent, absorbed water, oligomers, etc.) (1, 21). The melting temperature of PLA polymers depends also on many factors. Contrary to Tg, it can be modified by varying the contents in D- and L-lactides of the feed used for the synthesis. Stereocopolymers containing more than 90% of D- or L lactyl units are semicrystalline. Melting temperature of PLA 100 approaches a constant value of 184°C as the M of the polymer increases (21). The melting temperature and the heat of fusion decrease with incorporation of D-units (1). The melting temperature of semicrystalline PLA polymers is also affected by the distribution of L - and D-lactyl units, which depends on the copolymerization initiation and propagation stages and on the composition of the feed in L-, D- and mesolactides. It is worth noting that L - and D-lactyl units can be redistributed with more or less randomization due to transesterification side-reactions (22). Another means to modify the thermal properties of polymers is copolymerization combined with stereocopolymerization, occasionally. Several strategies have been developed to synthesize lactyl-containing copolymer chains : ring-opening copolymerization, polymerization initiated by macromonomers, and polymerization of asymmetric cyclic compounds consisting of two different units. Ring-opening copolymerization of lactides with other cyclic monomers results generally in random copolymers (23, 38). If the difference of comonomer reactivities is important and transesterification reactions are limited, copolymers with blocky structure are prepared (41). Real block copolymers can be obtained by successive addition of comonomers or by using a prepolymer as macroinitiator (55). Polymerization of dioxane-2,5-diones composed of two different hydroxy acids or morpholine-2,5diones containing one lactyl unit and one amino acid unit can give alternating copolymers (25, 64). Two main conditions must be fulfilled : one-side attack of the monomer ring and absence of transesterification reactions. These different strategies led to many copolymers (Table I).


w

Table I : Main copolymers of lactides Chemical nature

Comonomer

Polyester

Glycolide

Polyester

β-butyrolactone

Comonomer formula

(y

Distribution Refere nces of comonomer s random 23, 29 26 alternating random block


32 30,31

203 Table 1. Continued Polyester

γ-butyrolactone

random

33, 34

Polyester

δ-valerolactone

block

35

random block random

36, 38, 39 47 52

σex


0 °

Polyester

ε-caprolactone

Polyester

1,4-dioxane2,5-dione

Polyesterether

Ethylene oxide

V

block

55, 59

Polyesterether

l,5-dioxepane-2one

54

Polyesteramide

1,4-morpholine2,5-diones

a

random random alternating

66, 68 62, 64

random

70,71

random block

72 72

random

73

random

74

block

75

1

H Polyestercarbo nate

Trimethylenecarbonate

Polyestercaibo nate

2,2-dimethyltri methylenecarbonate

Polyestercarbo nate

2,2-[2-pentenel,5-diyl]trimethylenecarbonate

n Y O

V 0

O

Polyesteranhyd Adipic anhydride ride Polyestersiloxa ne

Polydimethylsiloxane

r < ÇH HO—(—Si—O—)—H 1 CH 3

x

3

The polymerization conditions influence directly the comonomer distribution in the resulting material and consequently the thermal properties. This paper reviews the most important linear degradable copolymers of lactic acid and their thermal


204 properties. Glass transition temperature data are primarily discussed with respect to temperature stresses usually found outdoor.


Polyesters

Poly(lactide-co-glycolide). Glycolide is the simplest cyclic dimer derived from α-hydroxy acids and the most common comonomer of lactides. Three methods are available to synthesize high molecular weight PLAXGAY. The first one is thering-openingpolymerization of a feed composed of lactide and glycolide. This method yields almost random copolymers. Initially, the difference of reactivities leads to preferential polymerization of glycolide. Later on, monomer units are usually redistributed and randomized, as a result of transesterification reactions (23). The second method is based on the ring-opening polymerization of 3-methyldioxane-2,5-dione, a cyclic dimer composai of one lactic and one glycolic acid units (24):

H

P-

Can the Glass Transition Temperature of PLA Polymers Be Increased?

Recommend Documents