Potassium Dichromate Surface Modification of Polyglycolic Acid (PGA)Multifilament

WANG Bi-qiao(王碧嶠)，ZHANG Pei-hua(張佩華) ，F(xiàn)ANG Mei-han(房美函)

1 Key Laboratory of Textile Science and Technology，Ministry of Education，Donghua University，Shanghai 201620，China

2 College of Textiles，Donghua University，Shanghai 201620，China

Introduction

Polyglycolic acid (PGA)is a thermoplastic polymer with hydrolytically labile aliphatic ester linkages［1］.Its most notable performances include its excellent fiber-forming ability，biodegradability， and controllable hydrolytic degradation.Because of excellent properties，it is widely used in the biomedical field，such as surgical sutures，drug delivery devices，and tissue engineering research［2-5］.However，its existing properties，such as hydrophily and biocompatibility，can not meet the demands of tissue engineering［6］.Therefore many researchers focus on surface modification of PGA［7-8］.A bio-adhesive glass，composed of SiO2，Na2O，CaO，and P2O5，has been used to modify PGA for bone engineering to increase microvascularization［7］.Sodium hydroxide is also used to pretreat PGA to enhance cell adhesion by breaking ester bonds and introducing carboxylic acid groups［8］.However， these approaches either decrease the mechanical properties of the polymer，or are complex.

Dichromate potassium (K2Cr2O7)is a common oxidant.Some researchers used K2Cr2O7to modify PE and PP for increasing surface activity［9］.In this study，we attempt easy operating and effective method to encourage the surface property of PGA filament，meanwhile maintain the good mechanical performance.Because the ester backbone can hydrolyze and the terminal hydroxyls may be oxidized，K2Cr2O7is attempted to encourage hydrolyzation of PGA filament.The effects of modification were examined，such as breaking force and water contact angle.Scanning electron microscopy (SEM) was used to examine the surface topography of the PGA surfaces before and after surface modification to identify whether the treatment caused morphological changes.Furthermore，differential scanning calorimeter (DSC) and Fourier transform infrared spectrometry (FTIR)were used to analyze the changes of molecular structure and functional group.

1 Experimental

1.1 Potassium dichromate surface modification

K2Cr2O7concentration of solution is 10%(m/m).H2SO4concentration of solution is 10% (m/m)，too.Two kinds of solutions were mixed with volume ratio 1∶6.PGA samples were dipped into mixed solution for different period:2，4，6，8，and 10 min;then cleaned by deionized water;and dried in a vacuum frozen dryer for 2 h.

1.2 Breaking force test

Breaking force of PGA filament was tested with LLY06E type electronic single fiber strength tester (Laizhou electron instrument Co.，Ltd.，China).Gauge length was 10 mm and stretch speed was 20 mm/min.The samples (n = 10)were tested with gauge length 10 mm，drawing speed 20 mm/min at 20℃，and relative humidity 65%.

1.3 Water contact angle test

Samples (n =5)of filament were pasted on the sample stage of contact angle tester (JC2000A，Shanghai Zhongchen，China)，and sprayed water.Shape of water-drop was observed and contact angle was recorded.

1.4 Surface topography

SEM (Hitach，S-4800，Japan)was used to examine the surface topography of the PGA filament surfaces before and after surface modification to identify whether the treatment caused morphological changes.

1.5 DSC test

The 3-5 mg fiber samples were placed in crucible，and their thermodynamic performances were tested by DSC (Pyris-1，PerkinElmer，US).Samples were kept warm for 2 min at 40℃，then heated up from 40 ℃to 250 ℃with a heating rate of 20 ℃/min.

1.6 FTIR test

The functional groups of unmodified and modified PGA were tested by FTIR (NEXUS670， Thermo Nicolet Corporation，US).

2 Results and Discussion

2.1 Tensile properties

The change of PGA multifilament breaking force can be seen in Fig.1.The breaking force curve of PGA rose a little at the beginning，and then dropped.It became stationary after 6 min.At the beginning，the effect of swelling was dominant.Swelling changed filament inner structure and made it uniform.Uniform structure may decrease inside stress and increase filament breaking force.With the modification time increasing，filament surface molecular chain hydrolysis played a leading role，which caused dropping of breaking force.Then two kinds of effects kept a balance after 6 min.Overall，the value of breaking force had not been changed a lot.It indicated the effects of K2Cr2O7surface modification concentrated on filament surface and had little impact on multifilament breaking force.

Fig.1 Breaking force with different K2 Cr2 O7 process times(values were expressed as means ± standard deviation(SD))

2.2 Water contact angle

In Fig.2 the water contact angles of PGA filament increased，then decreased from 2 min，and reaching the lowest point at 6 min.It increased once until 8 min，then decreased.

Fig.2 Water contact angle with different K2Cr2O7 process time (values were expressed as means ± SD)

Under the same test method and condition，two important influencing factors of material contact angle were material surface morphology and surface molecule polarity［9］.On one hand，surface modification can promote ester bond hydrolysis and increase of polar groups.This was beneficial to reduce the water contact angle.On the other hand，with the modification time increasing， filament surface roughness increased.However，contact angle and surface roughness didn't keep a linear relation.Depth of micropores had a critical value.When micropore depth was lower than the critical value，water contact angle increased.On the contrary，contact angle decreased if micropore depth was higher than the critical value［10］.The micropores amount also influenced the contact angle.At the beginning，micropores amount increased.With the modification time increasing，some nearby micropores may become big micropores.It influenced the micropores amount.Due to the joint effect of factors，the contact angle curves with different process time of PGA showed fluctuation.

Because decrease of contact angle meant improving of hydrophilicity，6 min treated PGA filament got the best hydrophilicity.In consideration of filament breaking strength，controlling process time as 6 min seems reasonable.

2.3 Surface topography

Figure 3 shows the SEM images of PGA filaments' before and after K2Cr2O7surface modification.PGA filament's surface only had some slight protrusion，but PGA filament of K2Cr2O7surface modification had obvious protrusion and depression.It proved the obvious effects on filament surface morphology change.

Fig.3 SEM images of (a)untreated PGA filament (b)K2 Cr2 O7 surface modified PGA filament

2.4 Thermal melting point (Tm)and crystallinity

Tmof PGA almost had no change after being treated，as the crystallinity decreased a little，from 53.60% to 52.05%，dropped about 1.55%.The drop of crystallinity was beneficial to multifilament hydrophilicity.The result showed that the chemical constituents and structure of PGA multifilament had little changes，and the effect of modification may concentrate on filament surface.It explained the reason of maintenance of filament breaking force after K2Cr2O7surface modification.

2.5 FTIR analysis

FTIR spectra of PGA before and after K2Cr2O7treated are shown in Fig.4.In untreated spectrum，1 741 cm-1wave number was CO absorption peak;1 087.7 cm-1was lactone C—O—C stretch absorption peak;1 154.2 cm-1was bridge ester C—O—C stretch absorption peak，1 417.4 cm-1was alkane CH2scissoring vibration absorption peak;2 961.2 cm-1was alkane CH2antisymmetric absorption peak;3 619.5 cm-1was intermolecular hydrogen-bonding O—H absorption peak.

Fig.4 FTIR spectra of PGA before and after K2Cr2O7 treated

Before and after K2Cr2O7treated，infrared spectra of PGLA filament had a little change.Absorption peak at 1 087.7 cm-1and 1 154.2 cm-1decreased， which indicated ester bond linkages on PGA filament surface may hydrolyze partly.Hydrolyzation of ester bond linkages indicated the hydroxylterminated and carboxyl end group may increase.The enhancement of intermolecular hydrogen-bonding O—H absorption peak at 3 619.5 cm-1supported it.Hydroxylterminated and carboxyl end groups are polar groups which are beneficial to increase hydrophilicity of filament.

3 Conclusions

In consideration of filament hydrophilicity and breaking force，the best K2Cr2O7surface modification processing time was 6 min.SEM pictures indicated modification had obvious effects on PGA filament surface morphology.Tmof PGA filament almost had no change，and crystallinity of PGA dropped a little，which meant effect of modification might concentrate on filament surface and structure of filament had little change.

FTIR spectra showed that hydrophilic groups，such as hydroxyl-terminated and carboxyl end group，increased in PGA filament.The result showed that the K2Cr2O7surface modification was an efficient method to improve PGA filament hydrophilicity and preserve its mechanical property.

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