semi-crystalline · semi-crystalline polymers: structure vs properties 9/23/2019 the information...

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SEMI-CRYSTALLINEPOLYMERS:

STRUCTURE VS PROPERTIES

9/23/2019The information presented in this document was assembled from literature of the resin product producer(s). The information is believed to be accurate however Entec Polymers (“Entec”) makes no representations as to its accuracy and assumes no obligation or liability for the information, including without limitation its content, any advice giv-en, or the results obtained. ENTEC DISCLAIMS ALL WARRANTIES, EXPRESSED OR IMPLIED, INCLUDING FITNESS FOR A PARTICULAR PURPOSE. The customer shall use its own independent skill and expertise in the evaluation of the resin. product to determine suitability for a particular application and accepts the results at its sole risk.


SEMI-CRYSTALLINE POLYMERS: STRUCTURE VS PROPERTIES


The combination of chain length, molecular weight, monomer unit combinations, tacticity, additive content, extent of branching and/or crosslinking, and lattice constants combine to determine a polymer’s molecular/physical properties. Each category can be modified by adjusting reactor temperatures, pressures, and catalyst composition. During injection molding and extrusion processes, the injection speed (or extruder output rate), melt temperature, mold/die temperature, and process pressure can influence the long range order and vary the degree of crystallinity. In general, the macromolecular form that polymer products take post processing can be referred to as the morphological structure; these exist as either amorphous, semi-crystalline, or liquid crystalline. The strongest correlation that leads to polymers with high crystallinity is the regularity of the structure of the polymer backbone and the flexibility of the polymer backbone. For these reasons atactic isomeric structures will have very low crystallinity while isotactic and syndiotactic materials will more readily recrystallize. This article will be focused on a few effects that degree of crystallinity and crystal size distribution can have on a semi-crystalline polymer’s chemical and mechanical properties. Polymer crystallinity is controlled by several parameters:

Rate of cooling post injection varies with cross sectional area as polymer chains align into a repeated structure.

Copolymerism: less energy is required to crystallize if monomer units are more consistently spaced – alternating and block copolymers can crystallize more rapidly as compared to random and graft copolymers.

Monomer complexity: crystallization is less likely in complex structures while simple polymers, such as polyethylene, crystallize relatively easily.

Chain configuration: linear polymers crystallize relatively easily while polymers with branches prevent rapid crystallization.

In general increased crystallinity brings higher density, more strength, and higher resistance to dissolution by chemical solvents, and higher resistance to softening by heating.




DETERMINATION OF CRYSTALLINITY

Degree of crystallinity can be determined with many analytical techniques. Below we show examples for DSC and density calculations.

DSC CALCULATION RATIO OF DENSITY

Base Equation % Crystallinity = X 100%

Required Constant (table lists enthalpy

of formation)

Experimental data shows enthalpy of

formation observed as 135.8 J/g

135.8/207 = 65.6%

ΔHobs

136.56 °C

ΔH°FF

Name Enthalpy(kJ/mol) Repeat Unit

Molecular Weight(g/mol)

Enthalpy(J/g)

PP 8.70 -CH2CH(CH3)- 42.08 207

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

0 20 40 60 80 100 120 140 160 180

Temperature (°C)

Hea

t Flo

w (W

/g)

125.58 °C

135.8 J/g

Base Equation: derived from first principles

total volume = volume amorphous + volume

crystalline and developed for the resulting form.

% Crystallinity = X%

Typical table of tabulated densities (amorphous

densities based on extrapolated data in the melt phase) – moisture in the sample can effect

measurement data if polymer is hygroscopic.

Percent Crystallinity = 1.004 (0.915 – 0.853) /

[0.915*(1.004 – 0.853)] = 45%

Example: Sample of PE has a density of 0.915. We know density of PE in the amorphous and crystalline state, plug into base equation to calculate

the percent crystallinity.

Pc (P-Pa)

P (Pc-Pa)

Polymer Da (g/cm3) Dc (g/cm3)

Polyethylene 0.853 1.004

Poly Vinyl Alcohol 1.291 1.350

Poly Vinyl Chloride 1.412 1.477

Poly Vinylidene Chloride 1.775 1.957

Poly EthyleneTerephthalate 1.336 1.514




Temperature Effect on Spherulite Growth Rate

Line

ar G

row

th R

ate

Temperature

120C 140C 160C 180C 200C 220C 240C

Melting Point = Intermolecular attraction overcome, molecules no longer oriented

Glass Transition Temperature = No

segmental motion for recrystallization

Simple Structure

Complex Structure

MELT/MOLD TEMPERATURE EFFECTS While spherulite radius growth is generally considered linear with time at a constant temperature, it is important to note that the injection molding thermal process is in constant fluctuation during the molten phase injection and subsequent cooling. During injection, molecular chains can become oriented and act as nucleation zones that other polymer chains build around. The initial re-crystallization from the melt will be slow until the material is below the melting point but above the glass transition temperature. Refer to the example shown to the right.

Monomer sub-units can alter the crystallinity for many polymers. The addition of altered monomer units changes the long range order that dispersion forces play on the polymer. This in turn reduces the crystallinity and eventually melting point.

MONOMER SUB-UNIT STRUCTURE

Radial Growth Rates (µm/min) for Various Compositions

Temperature(°C)

100%Isotactic

90%Isotactic

80%Isotactic

60%Isotactic

40%Isotactic

120 29.4 29.4 26.4 22.8 21.2

125 13.0 12.0 11.0 8.90 8.57

131 3.88 3.60 3.03 2.37 2.40

135 1.63 1.57 1.35 1.18 1.12

Melting Point (°C) 171 169 167 165 162

Table 1: Blends of unextracted Isotactic PP with Atactics PPThe addition of atactic PP to the matrix of isotactic PP reduces the melting point and radial growth rate of spherulites. This ultimately drives down percent crystallinity.

Table 2: PE Melting Point with VA ContentThe addition of Vinyl Acetate to PE helps to reduce the melting point and reduce crystallinity. This increases flexibility, cold temperature impact, and improves optical clarity of the product.

0 5 10 15 20 25 30 35 40

0

60

80

100

120

% Vinyl Acetate in Polymer (wt%)

Mel

ting

Poin

t (°C

) ▶

▶ ▶▶▶

▶ ▶

▶

▶▶▶




-30

Dar

t Im

pact

Ene

rgy

(Joul

es)

Density of Impact Modifier (g/cc)

IMPACT STRENGTH MODIFICATION As the density and percent crystallinity of olefinic elastomers increase, the efficiency of impact modification is reduced. Lower crystalline regions allow for mechanical energy to be absorbed by the mobility of chains in the polymer structure and prevent breakage. It is important to note that the operating temperature relative to an amorphous plastics’ Tg (glass transition) will influence the relative impact/elongation performance. Materials that are operating well below their Tg will behave in a glassy or brittle manner while materials above their Tg will have much lower mechanical strength while improving elongation at break.

Nucleation can affect the size and overall uniform geometry of crystals within a molded product. Rather than crystal formation occurring randomly, the nucleation sites act to initiate crystallization such that most spherulites are of the same order of magnitude (size). This helps reduce variations in the index of refraction and can help reduce haze in random copolymer PP and PE.

LATTICE SIZE / OPTICS

Table 1: DSC Curves for HDPE & HDPE Containing CRYSTALADD®

Nucleation PackagesExothermic recrystallization with nucleation agents helps to increase the onset temperature and peak recrystallization temperature for a polymer sample. This ultimately can reduce cycle time and reduce haze in resulting parts.

Table 2: Optical Haze (ASTM D1003) Utilizing Nucleation AdditivesUtilization of nucleation cells allow the molten polymer to contain uniform crystalline grain sizes. This uniformity variances in light refraction and reduces haze.

Hexene LLDPE

Haz

e (%

)

0.85 0.86 0.87 0.88 0.89 0.90 0.91

0 1

0

20

30

40

50

60

70

80

70 75 80 85 90 95 100 105 110 115 120 125 130°C

Control HDPE + CRYSTALADD®

114.67Control HDPE

20mW

0

2

4

6

8 1

0 1

2 1

4 1

6 1

8 2

0

Butene LLDPE Octene LLDPE

18.4

8.22

13.2

8.14

11.510.5

Control

Control + CRYSTALADD®

semi-crystalline · semi-crystalline polymers: structure vs properties 9/23/2019 the information...

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