| 查看: 2123 | 回复: 2 | |||
[求助]
请问什么是余辉长度(persistence length)？
|
|
请问什么是余辉长度(persistence length)？ [ Last edited by mengsk on 2013-3-11 at 14:58 ] |
回复此楼» 猜你喜欢
求助有石油化工中级工程师吗
已经有2人回复
-
寻环境工程中级一名，咨询问题
已经有0人回复
-
金属材料论文润色/翻译怎么收费?
已经有89人回复
-
我找下石油化工中级工程师咨询问题
已经有3人回复
-
淄博无机非金属材料测试
已经有0人回复
-
找可以唯一的化工工程，咨询问题
已经有2人回复
-
有化工工程中工吗，咨询问题
已经有0人回复
-
求助，请问有没有环境、环保中级工程师呀，咨询问题
已经有0人回复
-
有没有做非煤矿山相关工作，考安管证的呀
已经有1人回复
huzhifeng09
专家顾问 (著名写手)
-
专家经验: +327 - 应助: 176 (高中生)
- 贵宾: 0.178
- 金币: 6710.3
- 红花: 31
- 帖子: 2084
- 在线: 159.7小时
- 虫号: 2313969
- 注册: 2013-03-03
- 性别: GG
- 专业: 高分子科学
- 管辖: 高分子
2楼2013-03-12 11:59:53
|
Persistence length From Wikipedia The persistence length is a basic mechanical property quantifying the stiffness of a polymer. Informally, for pieces of the polymer that are shorter than the persistence length, the molecule behaves rather like a flexible elastic rod, while for pieces of the polymer that are much longer than the persistence length, the properties can only be described statistically, like a three-dimensional random walk. Formally, the persistence length, P, is defined as the length over which correlations in the direction of the tangent are lost. In a more chemical based manner it can also be defined as the average sum of the projections of all bonds j ≥ i on bond i in an indefinitely long chain.[1] Let us define the angle θ between a vector that is tangent to the polymer at position 0 (zero) and a tangent vector at a distance L away from position 0, along the contour of the chain. It can be shown that the expectation value of the cosine of the angle falls off exponentially with distance,[2] where P is the persistence length and the angled brackets denote the average over all starting positions. In polymer science jargon the persistence length is considered to be one half of the Kuhn length, the length of hypothetical segments that the chain can be considered as freely joined. The persistence length equals the average projection of the end-to-end vector on the tangent to the chain contour at a chain end in the limit of infinite chain length.[3] The persistence length can be also expressed using the bending stiffness , the Young's modulus E and knowing the section of the polymer chain.[4] In the case of a rigid and uniform rod I can be expressed as: where a is the radius. For example a piece of uncooked spaghetti has a persistence length on the order of m (taking in consideration a Young modulus of 0.1 GPa and a radius of 1 mm).[5] Double-helical DNA has a persistence length of about 500 Angstroms. |
3楼2013-03-13 15:15:09